Individuals with properdin, C3 or late complement component deficiency (LCCD) frequently develop meningococcal disease. Vaccination of these persons has been recommended, although reports on efficacy are scarce and not conclusive. We immunized 53 complement-deficient persons, of whom 19 had properdin deficiency, seven a C3 deficiency syndrome and 27 had LCCD with the tetravalent (ACYW) meningococcal capsular polysaccharide vaccine. Serological studies were performed in 43 of them. As controls 25 non-complement-deficient relatives of the complement-deficient vaccinees and 21 healthy non-related controls were vaccinated. Post-vaccination, complement-deficient individuals and controls developed a significant immunoglobulin-specific antibody response to capsular polysaccharides group A, C, Y, W135, but a great individual variation was noticed. Also, the proportion of vaccinees of the various vaccinated groups with a significant increase in bactericidal titre (assayed with heterologous complement) was similar. Opsonization of meningococci A and W135 with sera of the 20 LCCD individuals yielded in 11 (55%) and eight (40%) sera a significant increase of phagocytic activity after vaccination, respectively. Despite vaccination, four complement-deficient patients experienced six episodes of meningococcal disease in the 6 years post-vaccination. Four episodes were due to serogroup B, not included in the vaccine. Despite good response to serogroup Y upon vaccination, disease due to serogroup Y occurred in two C8beta-deficient patients, 3.5 and 5 years post-vaccination. These results support the recommendation to vaccinate complement-deficient individuals and to revaccinate them every 3 years.