We surveyed members of the Ocular Microbiology and Immunology Group and reviewed laboratory requests at the Centers for Disease Control to determine better the epidemiology of Acanthamoeba keratitis in the United States. A total of 208 cases of Acanthamoeba keratitis were identified. The number of cases increased gradually between 1981 and 1984, with a dramatic increase beginning in 1985. Males and females were equally affected. Of the 208 patients, 85 (41%) resided in California, Texas, Florida, or Pennsylvania. Of 189 patients, 160 (85%) wore contact lenses, predominantly daily-wear or extended-wear soft lenses. Of the 138 patients who wore contact lenses and for whom information was available, 88 (64%) used saline prepared by dissolving salt tablets in distilled water. Patients aged 50 years and older were more likely to have had a history of trauma than younger patients, and males were more likely to have a history of trauma than females.