A mother notified the Tazewell County Health Department, Illinois that her two sons had received a diagnosis of Cryptosporidiosis. The brothers had attended a private day-camp facility with a swimming pool and also participated in a day-camp outing to a community water park. A cohort study showed that entering the day-camp swimming pool or water park water increased the risk of diarrheal illness. Testing of water samples determined that the day-camp pool was negative for Cryptosporidium, and the water park was positive for Cryptosporidium parvum but negative for Cryptosporidium hominis. The implicated day-camp pool was closed, and the water park was hyperchlorinated.