This is a prospective study of the recent explosive epidemic of group A meningococcal meningitis that occurred in Nairobi from April 1989 to January 1990. It was compared with previous less explosive outbreaks that occurred from 1975 to 1981. The recent epidemic was correlated to its Neisseria meningiriais carrier status among case-control samples with carrier rate in nasopharyngeal respiratory pathogens of children born of human immunodeficiency virus (HIV-1) positive and negative mothers at about the same time of the outbreak. The effect of mass vaccination on the course of the outbreak is presented on selected affected areas of Nairobi city residents who were offered the bivalent meningococcal polysaccharide vaccine against serotype A and C