We have examined 125 individuals who have earlier had meningococcal (mgc) disease. They belonged to one or more of the following groups: (1) 2 or more cases of mgc disease in the same family; (2) individuals with 2 episodes of mgc disease or with 1 episode of mgc disease and 1 or more episodes of purulent meningitis of another aetiology; and (3) infections with Neisseria meningitidis belonging to serogroups that are uncommon as causes of disease and presumably low-virulent (W-135, 29E, X, Y). Among these we found 15 complement (C)-deficient individuals (12% The prevalence of C deficiency in the groups above was 7% 41% and 19% respectively. The first group (family cases), is very heterogeneous and may be further subdivided into 2 groups: families whose members fell ill within an interval of 30 days (in these the prevalence of C deficiency was 2% and families in which the interval between mgc disease cases exceeded 30 days (in those the prevalence of C deficiency was 14% We found a predominance of defects of the initiation pathways, with properdin deficiency being the most common.