A measles epidemic occurred in a rural West African village, despite documented immunization in 90% of the P < .001). Thus, the vaccine efficacy rate was 90.1%. The average age at presentation was 72 months, and older patients were more likely to have severe disease. Nutritional status had no effect on susceptibility to measles infection or on outcome; weight loss (350 ± 170 g) was most marked during the prodromal phase in young children. Infection was more severe when more than one case was diagnosed in a household (P < .05) and in conditions of overcrowding, which were also associated with higher infection rates (P < .02). Although immunization did not stop the spread of the epidemic within the village, a strict quarantine in the medical clinic probably prevented spread to other villages.