A severe epidemic of group A meningococcal disease occurred in the northwest part of the Central African Republic from January to March 1992. The outbreak affected a large and densely populated area, with a poor road network, located 400 kilometers south of the classical meningitis belt. An initial selective vaccination campaign was carried out by the national health care service. As the epidemic was continuing, the national authorities asked for international assistance. The French participated by sending Bioforce, a medical task force designed by the Ministry of Defense, with the financial support of the Ministry of Cooperation. Neisseria meningitidis strains were isolated and identified within 36 hours by the Bioforce field laboratory. Strains from 24 patients were sent to the Pasteur Institute in Paris (Neisseria Unit) for serotyping, testing of antibiotic susceptibility, and multilocus enzyme electrophoresis. With one exception, all strains had formula A:4:P1.9. By an initial rapid assessment, the limits of the affected area and populations were determined. The weekly incidence rates observed in different areas varied within a range of 3 to 10 cases per 1,000, with fatality rates from 20 to 30 cases per 100. The spread of the epidemic was stopped by a mass vaccination campaign, which targeted the entire population (200,000 immunizations) of the affected area. The case fatality rate could not be reduced below 15%, despite antimicrobial treatments implemented as soon as possible. The optimal treatment was the standard single intramuscular injection of oily chloramphenicol. The predictive values of clinical symptoms were calculated. The efficacy of vaccination was estimated by comparison of the percentage of people immunized and the proportion of those vaccinated people who developed meningitis identified during the 3 weeks following the mass vaccination campaign. The efficacy varied between 93 to 95% according to the place. The typical weekly incidence rate of 1 case per 1,000 is not a relevant threshold to sufficiently and immediately detect a meningitis outbreak and needs to be reconsidered.