A passerby spotted a seemingly incoherent man, identified as Ramiro Reynoso, sitting in a parked but running car Sunday morning, and called for help, the Chicago Tribune reported.
Reynoso was place in an ambulance and transported to Resurrection Medical Center.
Chicago Fire Department spokesman Larry Langford said that at some point during the ride, Reynoso, who was strapped in a gurney, told the medics that he needed to vomit.
"So they would have handed him the vessel and unbuckled the top strap as he sat up in order to deal with it," Langford said.
After the top straps were undone, Reynoso unstrapped his legs and jumped out the back of the moving ambulance, Langford said.
Reynoso suffered major head injuries and was pronounced dead at 6:40 a.m. at Resurrection, the medical examiner's office said.
"This is a tragedy," Langford said. "Our sympathies go out to his family."