A case-control study was conducted in which the amount and sites (fresh-water lakes and rivers, chlorinated pools, or the ocean) of recent swimming by 105 patients with otitis externa were compared with that of 239 controls. Swimming during the week prior to the visit was strongly associated with otitis externa. When the 80 cases and 127 controls with a history of recent swimming were compared, otitis externa was positively associated with the amount of swimming during the preceding week. Otitis externa was also positively associated with swimming in fresh water compared with ocean or pool swimming with the magnitude of this association being more pronounced at higher levels of exposure.