Methods.
—We conducted a mail survey of a probability sample of Sacramento County, California, households with in-ground swimming pools in January 1988; 80% of eligible subjects responded. Onset of exposure to a pool was defined as the month in which (1) the responding household had a pool installed, or (2) the responding household first occupied a residence with a pool. Exposures began in 1959 through 1987.

Results.
—The home swimming pool immersion event rate was 11 per 1000 pool-years. The rate per 1000 pool-years was higher for the first 6 months of exposure than thereafter (0 to 6 months, 44; 7 to 24 months, 14; >24 months, seven), but 77% of events occurred outside the high-risk period. For households whose exposures began in 1984 through 1987, the immersion event rate was 51 per 1000 pool-years overall and 123 per 1000 pool-years for the first 6 months of pool exposure; these increases probably represent underreporting of earlier events. In this group, 48% of events occurred outside the high-risk period. The family swimming pool accounted for 91% of immersion events at the respondents' homes.

Conclusions.
—The residential swimming pool is an important hazard for pool-owning households. The first 6 months of exposure constitute a high-risk period, but many immersion events occur later. Pool drowning prevention programs may focus on newly acquired swimming pools and their owners but should be as broad as possible to maximize their effectiveness.

(AJDC. 1991;145:1200-1203)