BACKGROUND. Washington State birth certificates were used to conduct a population-based case-control study to assess the possible association of maternal smoking with polyhydramnios. METHODS. All singleton births complicated by polyhydramnios (n = 557) were identified from the vital records for the years 1984 to 1987. For comparison, 1671 records were randomly selected for the same years from singleton births uncomplicated by polyhydramnios. RESULTS. Women who reportedly smoked prenatally were found to be at increased risk for polyhydramnios (relative risk [RR] = 1.7, 95% confidence interval [CI] = 1.5-2.1, adjusted for marital status, maternal age, and parity). When women with conditions known to be associated with polyhydramnios were excluded, the risk for those who smoked prenatally remained elevated (RR = 1.8, 95% CI = 1.1-2.3). CONCLUSION. Overdistention of the uterus from polyhydramnios may cause a variety of pregnancy complications. The observed association of smoking with polyhydramnios may be a further indication for public health interventions aimed at preventing smoking during pregnancy.