The rain held off for the 7:30 a.m. start, but race organizers raised the alert level to yellow, indicating some precautions need to be taken. Weather was a factor for runners throughout the day, with showers and high humidity during the race and slick spots along the route.
"Temperatures may fool them. The humidity is going to be something that may make it feel a little bit warmer than the temperatures," said Dr. George Chiampas, medical director of the Chicago Marathon, ahead of the race.
Several changes were made to the race course, which will weave through 29 neighborhoods on the city's North, West and South Sides. To view the full course map, click here.
CHICAGO MARATHON 2018 WINNERS
Britain's Mo Farah won the Chicago Marathon on Sunday, claiming his first marathon victory in three attempts.
Farah finished in 2 hours, 5 minutes, 11 seconds. It was the first marathon on U.S. soil for Farah, who's won four Olympic gold medals and six world championships on the track.
Mosinet Geremew of Ethiopia finished second, 13 seconds behind Farah. Japan's Suguru Osako came in third at 2:05:50. American Galen Rupp, last year's winner and Farah's former training partner, was fifth at 2:06:21.
Kenya's Brigid Kosgei won the women's race in 2:18:35, the third-fastest time in the history of the Chicago race. Kosgei finished second in Chicago last year.
Ethiopia's Roza Dereje finished second, 2 minutes and 43 seconds behind Kosgei. Shure Demise of Ethiopia was third at 2:22:15.
The Associated Press contributed to this report.