From June 1 to August 31, 1993, 317 cases of swimmer’s itch were reported by the residents of Walloon Lake. The highest number of cases occurred between June 29 and July 5 when 161 individuals reported acquiring swimmer’s itch. Individuals who acquired swimmer’s itch were younger than controls selected from the population. Other risk factors for development of swimmer’s itch included swimming in the North Arm, swimming on and off throughout the day and swimming close to shore. Water skiing and tubing were associated with a decreased risk of acquiring swimmer’s itch. There was no support for previous reports that swimming early in the morning increased the risk of acquiring swimmer’s itch. While past reports have suggested that bright sun triggers cercarial shedding therefore increasing the risk of developing swimmer’s itch, days with little sky cover did not have higher numbers of cases than did overcast days. The number of cases per day was surprisingly unrelated to the proportion of individuals exposed to lake water. The number of cases was, however, independently associated with the average water temperature and maximum air temperature.