Helium balloons get loose, and either get stuck in trees and make the park look sky littered, or fly out to sea and kill sea turtles when they eat them (true story).
So, my guess is an eco-friendly city council has decided to ban fun in the park.

Cart has hit the nail on the head: the ban is for environmental reasons. Helium balloons are completely environmentally irresponsible. I saw one floating around the other day and said a prayer for the bird it was eventually going to kill. Sure, it might look cool, but eventually the balloon is going to come down and more likely injure or kill that bird or animal than just become liter.

Carto, your reason sounds like the real reason. Let's label that one number 5: environmental reasons. Let's consider another reason, #6: Bean resemblance.
Anything that looks remotely like the Chicago Bean is banned from the park. Balloons kinda look like a bean, thus are banned.

Tom, do helium balloons floating in the sky actually kill birds? Does a bird see the balloon and think, "Gee, wouldn't it be fun to fly into that object in the sky?" I guess if it's nighttime, there's a chance the bird won't see the balloon and fly into it. Birds do that with buildings way too often, but don't birds fly into the buildings because of--oh crap, what is it? The lights? That's not it. There's some strange reason birds fly into buildings. The wind currents or something?

Even professional ornithologists don't really know why birds fly into windows [more common than flying into the building itself] though some people suggest that birds don't see reflections and that they see windows as a portal. Oddly enough some buildings are more susceptible to having birds fly into their windows than others [flight patterns?].

My mother has a neighbor who has a black construction paper bird on her window as a detractor to her window's unusual penchant for attracting birds. My building downtown also attracts the poor things. At first I thought my weekly sighting of a dead bird on the sidewalk was West Nile until I realized the poor things were doing themselves in. As far as the balloons, birds are more likely to suffer their fate after the balloon has descended into a tree or hit the ground [deflated]. The spent balloon is most-likely to kill them by suffocation.

Could also be that some people fill balloons with nitrous oxide and get high. They're called whip-its. Not sure why someone would go to the trouble of bringing a nitrous-filled balloon to Millennium Park though.

Speaking of balloons, did anyone see the massive amount of Mickey Mouse balloons they were giving out in Pioneer Court yesterday? I totally should have gotten one. Argh!

Lisa S, let's make your reason as reason #7: covert nitrous oxide balloons.

There seems to be a huge amount of speculation regarding balloons and not evidence regarding the green issues. Banning Balloons seems a little harsh. Can the Balloon manufacturers and people who are concerned about the environment get together and talk to see if an environmental balloon can be produced ? Banning things is not t he answer.

There is a helium shortage too; though I don't see how banning them in Millennium Park would positively or negatively affect that. The park would have to sell the balloons and then ban the sale. That's what makes me thing the ban is an environmental issue. Are helium balloons banned in all Chicago parks?

Should we ban candy bars in the park? Because people leave the wrappers on the ground. Wait, wait. Let's ban plastic bags. Because people could accidentally drop one, and they get stuck in trees.

Regular readers of this blog know that I'm not a fan of plastic bags. In fact, I LOVE reusable bags. (I need to blog about them, I only mentioned them in a comment on this envrironment post.) But just because I think plastic bags are dumb doesn't mean that people shouldn't be able to carry them around. The likelyhood of someone dropping a plastic bag and it getting caught in a tree or sitting in the curb is just as likely--if not more likely--than a helium balloon getting stuck somewhere. Think about it. How many plastic bags have you seen littered about. How many helium balloons have you seen llittered about? I would dare to say there are ten times more plastic bags littered about than helium balloons. No wait. I would say fifty times more. I can't even remember the last time I saw a helium balloon sitting caught somewhere.

Helium balloons bring joy and happiness--thus should be allowed. Plastic bags do not bring happiness. They only encourage laziness. Yet we allow plastic bags EVERYWHERE. And i mean EVERYWHERE. You are considered kinda strange when you don't want a plastic bag to carry your candy bar home. I bought some toilet paper this weekend, and frick it, I just carried the toilet paper without a bag. Yes. WITHOUT A BAG.

I ALLOWED MY TOILET PAPER TO BE SEEN BY THE AMERICAN PUBLIC.

In fact, I tossed my toilet paper up in the air many times and played catch with myself on my way home.

I WAS FREE FROM THE PLASTIC BAGS.

Long live helium balloons, AND DOWN WITH PLASTIC BAGS.

I never understood why people are so ashamed of buying and/or carrying around toilet paper. We all use it! It's a fact of life. Some women have similar issues when buying feminine products, but that's probably a subject for another blog :-D

You are TOO funny, spudart! I love it! I often refuse a bag when I'm only purchasing one or two items [especially if I don't have my reusable bag(s) with me].

I think Da Mayor should ban plastic bags in Chicago the way that San Francisco has. I like that Whole Foods has done away with them completely, but I think we should broaden our efforts.

Funny that you mention that you never posted about reusable bags. I thought you had ... but it was an email you sent around!

I think I'm ready to buy some more. I've given half of mine away in hopes of spreading the word about reusable bags!

Oh, and people ... please don't litter the ground with the wrappers from your Chupa Chups and other candy! ;o)

If Chicago is so concerned about pollution, they should address visual pollution and get rid of that hideous Cloud Gate®, as well as the new bandshell and that incongruous Greek temple. And while they're at it, also deep-six those "2001: A Space Odyssey" monoliths that pee all over tourists.

Oh, and screw Whole Foods.

Leigh! On a rampage! You are like Godzilla tearing down Chicago landmarks in one fell swoop. I'm trying to think of one I'd like you tear down for me, but I'm having a hard time.

Oh can you just tear off the top of the R R Donnelly Building? I'm not a fan of the greeky style roof on a skyscraper. Actually, it's not even called the RR Donnelly building anymore. RR Donnelley moved their corporate headquarters to a different location in May 2005, and in 2007 the building became the corporate headquarters of United Airlines. (I didn't want you tearing off the top of the wrong building, thanks).