In June 1982, an outbreak of gastrointestinal illness caused by Shigella sonnei occurred among residents of two counties in Oklahoma. A case-control study of cases and age and sex-matched controls showed an association with attendance at a southern Oklahoma lake (14/17 cases vs 3/17 controls, matched pair odds ratio [OR] 9/0, confidence interval [CI] 2.4-infinity). A survey of 85 persons who had visited the lake area showed that persons who had swum were more likely to have been ill with a gastrointestinal illness