An epidemic of type 1 poliomyelitis involving 305 cases occurred in The Gambia (estimated 1986 population, 768,995) from May through November 1986, following a 6-year period when only five cases were reported. Cases were identified by physician reporting during the epidemic and by a national village-to-village search conducted after the epidemic. The national attack rate was 40 cases per 100,000 people. Cases lived in all parts of the country except the capital, Banjul. The peak month of the epidemic was August (139 cases). The highest attack rate by year of age was in 1-year-old children (394 cases per 100,000 persons), and 75% of cases were 3 years of age or less. A vaccination coverage survey showed that 64% (95% confidence interval 60-68) of 1- to 2-year-old children were vaccinated with at least three doses of trivalent oral polio vaccine at the beginning of the epidemic. Fifty-seven cases became paralyzed more than 2 weeks after a national mass campaign in which 95% of children 1-7 years old were reported to have received a dose of trivalent oral polio vaccine. Experience in The Gambia shows that a several-year period of excellent control of endemic poliomyelitis by a vaccination program can be followed by a major epidemic and that a mass vaccination campaign may be only partially successful in ending the epidemic.