Friday's high midday heat and humidity kept emergency crews scrambling during the Blackhawks' Stanley Cup parade and rally with as many as 125 people needing help due to heat exhaustion.
One officer said people were "dropping like flies."
Twenty suburban ambulances were called in to assist the Chicago Fire Department deal with collapsing fans.
Fifty people were taken to area hospitals, mostly for heat-related illnesses, which were not considered to be life-threatening. Northwestern Memorial reported receiving more than 20 fans.
"The number of people being treated is very small compared to the total number of people attending the rally," said Fire Media Affairs Dir. Larry Langford.
The city provided water and cooling areas during the festivities and Chicago Transit Authority buses were set up on Lower Wacker Drive.
"Many of the people, once they're taken downstairs to get hydrated and are allowed to sit for a minute, they just go right back up into the celebration," Langford said. "Some people just need a break to recover a little."
The Salvation Army and several other organizations are also dispersing water throughout the crowd.
The crowd estimate for the celebration was more than 2 million, according to OEMC spokeswoman Therese Kordelewski.
As of noon, the temperature was recorded at 85 degrees near Midway Airport with the heat index at 91 degrees, according to the National Weather Service.
A Northwestern Memorial Hospital spokeswoman said that with the all the festivities going on in Chicago this weekend, she expects to see more heat-related cases.