To examine whether hot tub or whirlpool bath use during pregnancy increases the risk of miscarriage, the authors conducted a 1996-1998 population-based prospective cohort study at the Kaiser Permanente Medical Care Program in Oakland, California. Of 2729 eligible women, 1063 completed the interview. Miscarriage before 20 weeks of gestation was ascertained for all participants. Information on hot tub or whirlpool bath use was obtained during an in-person interview conducted early in the pregnancy. A Cox proportional hazards model was used to estimate the hazard ratio after adjustment for potential confounders. Compared with nonuse, use of a hot tub or whirlpool bath after conception was associated with a twofold increased risk of miscarriage (adjusted hazard ratio (aHR) = 2.0, 95% confidence interval: 1.3, 3.1). The risk seemed to increase with increasing frequency of use (aHR = 1.7 for less than once a week, aHR = 2.0 for once a week, and aHR = 2.7 for more than once a week) and with use during early gestation (aHR = 2.3 for initial use within the first 4 weeks of the last menstrual period and aHR = 1.5 for initial use after 4 weeks of the last menstrual period). Findings suggest an association between use of a hot tub or whirlpool bath during early pregnancy and the risk of miscarriage.