An outbreak of acute gastroenteritis involving 249 persons, 32% of whom were hospitalized, occurred on a U.S. Army installation in 1990. Campylobacter jejuni was isolated from 81 of 163 (50%) persons cultured. Seventeen isolates of C. jejuni available for serotyping were Lior serotype 5. The outbreak remained restricted to one recruit barracks area and adjacent Junior Reserve Officer Training Corps cadet barracks. Infection of sequential cohorts of recruits over an interval of 3 weeks suggested a continuing or intermittent common source. Contaminated food was not implicated because affected persons ate at separate dining facilities and other facilities with the same food sources had no associated illnesses. There was a strong association between the amount of water consumed by recruits and risk of diarrhea (chi-square test for trend, p<0.001). Samples of drinking water collected in the affected area had no residual chlorine and when cultured yielded greater than 200 colonies of coliform bacteria per 100 mL of water sampled. Although Campylobacter was not isolated from water, living and dead birds were found in an elevated water storage tank providing drinking water to the affected area. This and other similar outbreaks indicate that contamination of water storage tanks can lead to large outbreaks of Campylobacter enteritis.