A prospective population-based field study on Haemophilus influenzae type b (Hib) meningitis was carried out in The Gambia, West Africa. The annual incidence was 60 cases/100,000 children less than 5 years old and 297 cases/100,000 less than 1 year. The peak incidence was in those 5 months old, and 45% of cases occurred in those less than 6 months. The case fatality rate was 37%. Because the outcome did not appear to be affected by the logistics of receiving care, and resistance to chloramphenicol and ampicillin is not yet a problem, prevention of Hib disease at an early age seems the best solution. There are several options for vaccine trials. On the basis of available data, at least one of these trials seems warranted before Hib vaccination can be introduced in developing countries. Extrapolation of successful results in industrialized countries to a population of different genetic origin and living under different conditions seems questionable.