CHICAGO — In scorching heat and high humidity, the Chicago Marathon took a deadly turn Sunday.

One runner died, at least 49 were hospitalized and thousands were denied the chance to cross the finish line in the race long known for its brisk fall temperatures and flat terrain.

A runner from England, Paul Gardiner, said the weather made for a “brutal” run.

“We were at about 18 miles and we heard they canceled it and that kind of sent a little bit of concern through the crowd,” he said. “It’s just it’s impossible to run.”

The 88-degree heat and sweltering humidity were so draining that organizers shut down the second half of the course 31⁄2 hours after the start.

About 10,000 of the 45,000 registered runners chose to not race in the heat despite more mist stations, cooling buses, and water-soaked sponges, while another 10,934 started but didn’t finish, officials said.

Kenya’s Patrick Ivuti won, leaning at the finish line to edge Jaouad Gharib of Morocco by 0.05 seconds. Ethiopia’s Berhane Adere rallied to successfully defend her women’s title.

An autopsy is scheduled for Monday for Chad Schieber, 35, of Midland, Mich. He collapsed while running on the South Side and was pronounced dead shortly before 1 p.m. at a Veteran’s Affairs hospital, the Cook County medical examiner’s office.

“Obviously very sad news, and our thoughts and prayers are with the individual’s family,” a senior vice president of title sponsor LaSalle Bank, Shawn Platt, said.

The race’s medical director, George Chiampas, said witnesses reported seeing Schieber collapse and become unresponsive. “It sounds like he lost his pulse very fast and died on the race course,” Mr. Chiampas said.

These were record temperatures for the Chicago Marathon, topping the mark of 84 degrees in 1979. Runners were diverted to the starting area, where they were provided with medical attention and cooling misters. Shortages of water and energy drinks were reported along the 26.2-mile route.

Race director Carey Pinkowski said organizers were concerned that emergency medical personnel wouldn’t be able to keep up with heat-related injuries.

“We were seeing a high rate of people that were struggling,” Ms. Pinkowski said. “If you were out there at 1 o’clock, it was a hot sun. It was like a summer day. It was just a brutally hot day.”

At first, organizers hoped those who passed the halfway mark could complete the run. But eventually even those recreational runners were told to turn back.