An outbreak of Leptospirosis occurred among athletes who participated in a triathlon in Springfield, Illinois. For one segment of the event, athletes swam in Lake Springfield, a manmade lake. Swallowing lake water was a risk factor for illness among the athletes. Laboratory confirmation of infection was obtained from 52 athletes. A lake water sample suggested the presence of Leptospira. A survey of local residents was performed to determine if residents had leptospirosis. Two hundred ninety three suspect Leptospirosis infections were found in the community; twelve of these were subsequently confirmed by laboratory methods. Heavy rains preceded the triathlon which likely increased contamination of Lake Springfield.