Disease investigators at the Minnesota Department of Health identified a total of 69 people who became ill with diarrhea after swimming at Lake Nokomis. Cases became ill after swimming at the lake between July 16 and August 11. Four persons were laboratory-confirmed with E. coli O145:H28 infections. two factors significantly associated with illness were swallowing lake water (odds ratio = 3.80; 95% CI = 1.17–12.38) and age ≤10 years (odds ratio = 2.90; 95% CI = 1.57–5.35). No hospitalizations or cases of hemolytic uremic syndrome were reported.