An outbreak of Cyclosporiasis was epidemiologically linked to the consumption of tap water within a physicians' dormitory at a hospital in Chicago, Illinois. Drinking tap water in the physicians' dormitory or attending a housestaff association party held on June 29 in the physicians' dormitory were significantly associated with illness. The water storage tanks were not sealed from environmental contamination, and the stagnant zone at the bottom of the tanks could have permitted the growth or multiplication of microorganisms. After the water system pump had failed and the storage tank was emptied, the water at the bottom of the tank likely was stirred up when the pump was fixed and water splashed into the tank. This was the first reported outbreak of diarrheal illness due to Cyclospora in the United States.