People started becoming ill about an hour after eating a turkey dinner Sunday, The Denver Post reported.
Lt. Phil Champagne, a spokesman for the Denver Fire Department, said about 350 people ate the dinner, and 40 became "violently ill."
Some 14 ambulances transported those who became sick to area hospitals.
Champagne said now the main concern is that many left the mission after dinner and may be sick on the streets or the alleys.
"We've started getting calls already, about four or five -- relatively nearby -- about folks getting really sick," Champagne said. "They are starting to get dehydrated from all the loss of fluids."
Denver Rescue Mission spokeswoman Alexxa Tavlarides said the mission is trying to figure out the source of the contamination, but said the dinner was immediately pulled off the serving line.
"It sounds like we may have had some donations today, which is fairly common, so we're looking into that," Tavlarides said.