Two epidemiological studies of otitis externa were conducted, one was a prospective study conducted in the summer of 1979 which compared boy scouts at camp that swam in a fresh water lake with boy scouts at another camp that swam in a chlorinated swimming pool, and the other was a retrospective study conducted at Yale University during the summer of 1980. The prospective study yielded no cases of otitis externa; the retrospective study compared 29 cases with 29 controls who were matched by age and sex. In the prospective study a higher relative humidity at the camp where the boy scouts swam in a chlorinated pool was associated with a greater degree of abnormal flora colonization of the ear (Gram-negative bacteria and S. aureus). In the retrospective study, positive association was demonstrated with cases of otitis externa for ambient air temperatures, water temperature, less than 18 years of age, being female, swimming and length of time spent swimming. There was no association between cases of otitis externa and water quality as measured by fecal coliforms enterococci and P. aeruginosa and between abnormal flora colonization of the ear with bacterial indices of water quality.