Cercarial dermatitis (swimmer's itch) is an annoying inflammatory response to penetration of skin by non-human schistosome parasites while swimming or wading in lakes. The purpose of this survey was to identify risk factors for cercarial dermatitis and to determine whether limnological characteristics of the lake were associated with development of the condition. Starting 1 June 1993, an active case surveillance system was established in the communities around Walloon Lake, Michigan, which continued until 30 August 1993; controls were randomly selected weekly during the same time period through telephone surveys. A total of 317 cases of cercarial dermatitis occurring in 146 clusters was identified over the course of the summer and 295 controls were selected from the weekly telephone surveys. Factors associated with the development of cercarial dermatitis identified in the study included age, the time of day at which exposure to lake water occurred, and the month in which exposure to lake water took place. In addition, development of cercarial dermatitis was significantly associated with exposure to lake water in the area of the lake with the highest algae content and shallowest depth.