My parents are traveling from Mexico to London but theres a change flight in Chicago, do they need a USA visa?

Since they are not visiting USA and it is just a flight change I think they don't need it, but I am not sure. Please help me!

Their regular visa should be the only thing they need. When I go to Europe my USA visa works fine.

Nope they do not, when they land they are still on international grounds, even inside the terminal :)

They of course need their native visa, but anyone knows that.

Once you get inside passed the metal detectors and security at ''airport A'' you're on international ground, no matter what country. You only need an American visa if your parents would actually have the US as their final destination and actually leave the terminal completely.

As of last October the Transit without Visa program was still suspended (it was suspended in 2003 as a result of security measures after 9/11.)

I had heard it was reinstated, but I cannot find any information on the state department's web site to say that this is true.

Your parents should check with the US Consulate in Mexico.

YES! They need a VISA!

They need a "C-1" Visa or "Alien in transit directly through U.S."

see:

http://www.uscis.gov/portal/site/uscis/menuitem.5af9bb95919f35e66f614176543f6d1a/?vgnextoid=ad21f6670a092110VgnVCM1000004718190aRCR D&vgnextchannel=7913ffa920b82110VgnVCM100000471819 0aRCRD

The Transit Without Visa program (TWOV) and the International-to-International transit program (ITI), have been suspended. The action takes effect at 11:00 a.m., Saturday August 2, 2003!!!

see:
http://www.dhs.gov/xnews/releases/press_release_0227.shtm

Hurry-up, as it take quite a-while! Or fly DIRECT via BA/Bristish Airways or via CDG/Paris wirg AF/Air France or via MAD/Madrid with IB.Iberia!!!