Seabather's eruption is an annoying pruritic dermatitis that appears on the areas covered by the bathing suit as an erythematous macular or papular dermatitis, with or without urticaria. It occurs sporadically in Florida, the Caribbean, and as far north as Bermuda. The cause is not known.We collected information in Nassau County, Long Island, New York, about cases of all types of water-related dermatitis reported by beach personnel, health providers, and affected swimmers from 1970 through 1991. Concurrently, we surveyed all Nassau County swimming waters, especially during the summer season (June through September), for the presence of organisms capable of causing dermatitis. In 1980 a sudden epidemic of a severe, unfamiliar dermatitis in ocean bathers prompted increased surveillance of cases and waters. Planula larvae of the phylum Cnidaria were collected from the ocean and beaches and inside bathing suits. They were examined, photographed, tested on healthy subjects, and observed in the laboratory for metamorphosis.Three outbreaks of seabather's eruption have occurred on Long Island since 1975. The first, in August 1975, affected a small number of swimmers on the eastern end of Long Island. In 1980 there were thousands of cases along the entire south shore of the island, and in 1990 there were hundreds in the same area. In nonepidemic years, five or fewer cases have been reported yearly. Surveillance for larvae revealed them to be present during the swimming season in epidemic years, but in nonepidemic years they did not appear until autumn, after the swimming season. Applying larvae to the skin of healthy subjects produced a dermatitis indistinguishable from seabather's eruption. All larvae metamorphosed in the laboratory to the adult sea anemone Edwardsiella lineata.Seabather's eruption, previously reported only as sporadic cases in southern climates, has occurred sporadically and in outbreaks 1000 miles north of most previously described cases. These Long Island episodes were probably caused by the planula larvae of E. lineata.