Large meningococcal group A epidemics occur periodically in the Sudan, a country within the "meningitis belt" of Sub-Saharan Africa. Immunization with meningococcal polysaccharide vaccine induces protective serum bactericidal titers but little information is available on the duration of protection. Serum samples were obtained from 20 subjects, aged 11-47 years, who resided in the Sudan, and who had participated in a meningococcal polysaccharide immunogenicity study five years earlier. Persistence of serum group A bactericidal titers (measured with human complement) was compared to that of 12 immunized adults in North America with no known exposure to group A organisms. One month after vaccination, there were no significant differences in the serum bactericidal titers of the two groups. By five years the respective reciprocal geometric mean bactericidal titers had declined in both groups (82 to 34 in Sudanese, and 69-11 in North Americans, p < or = 0.03). However, the proportion of sera with protective bactericidal titers (> or =1:4) at five years was higher in the Sudanese than North Americans (80% vs. 42%, p < or = 0.05). Recommendations for periodic meningococcal polysaccharide vaccination every 3-5 years to maintain group A immunity may be more appropriate for persons living outside of endemic areas than for persons residing in endemic regions since immunity in endemic areas can be maintained by periodic exposure to group A organisms, even during periods between epidemics.