More than 550 health institutions and professionals endorsed the letter, addressed to Oak Brook-based McDonald's CEO Jim Skinner.
"We're rallying behind a simple message to McDonald's: Stop making the next generation sick," Dr. Steven Rothschild, Rush University Medical Center, said.
"I think it's time McDonald's stopped using the $10 million it spends on charity to defend the $400 million it spends marketing to kids," Deborah Lapidus, director of Corporate Accountability International, said.
The group also wants the company's longtime character Ronald McDonald to retire.
"The clown lets you believe that he's harmless, that he is neutral, and he is smiling, and he's dressed in pretty clothes, and well, what child doesn't -- isn't attracted to that kind of thing? So there is all kinds of gimmicks that are used to entice a child when the parents are not around," Doctor Alfred Klinger said.
McDonald Corporation spends over $400 million on ads targeting children, according to the activists.
"I don't think fast food should be their choice every day. I think it is pretty good for them to get away from kids a bit," Vito Guarino, parent, said.
Most parents with whom ABC7 spoke didn't see a need to retire the clown and thought the choice should be left to parents.
"Parents need to take control of kids eating fast food. If they want fast food it has to be in moderation," another parent said.
"All parents should limit their intake of fast food for children once in a while as a treat, not on a day-to-day basis," said a third parent.
McDonald's did not respond to a request for a statement or interview. The corporation has an annual shareholder meeting Thursday.