An epidemic of poliomyelitis caused by poliovirus type 1 occurred in The Gambia in 1986. To determine if a relationship existed between the failure of trivalent oral poliovirus vaccine (OPV) to prevent poliomyelitisand the season when children were vaccinated, 46 children 1–7 years old with poliomyelitis who had received three card-documented doses of OPV were compared with 260 controls who had also received three card-documented doses. Controls were individually matched with children who had poliomyelitis by age, sex, and residence. Children with poliomyelitis were more likely to have received doses in the rainy season (odds ratio describing the linear trend of each additional dose in the rainy season, 1.7; 95% confidence interval, 1.05–2.9). This finding extends previous observations of seasonal difference in the immunogenicity of OPV in The Gambia by showing that season of administration was associated with increased risk of vaccine failure nationwide for a several-year period.