Data from a 1979 measles epidemic in an urban district of Guinea- Bissau indicate that state of nutrition is not a major determinant of outcome of infection. However, overcrowding increases the risk of early infection and the severity of disease. In instances in which several children have measles simultaneously, the case fatality rate is significantly higher than for isolated cases. This tendency is apparently a result of intensity of exposure; within the same house, secondary cases have a much higher age-specific case fatality rate than index cases. It is suggested that the association between intensive exposure and severity of infection may be due to increase rates of intercurrent infection and/or a greater dose of infection. Since it is not only the malnourished children who die of measles, vaccination may have a greater importance for survival patterns than has previously been assumed.