Congenital complement deficiency states occur very rarely. These deficiencies are associated with a high risk of meningococcal disease (MD). We suggest that the following groups of individuals with MD are examined for complement deficiencies: 1. Individuals belonging to families, in which more than one case of MD has occurred with an interval exceeding one month. 2. Individuals infected with the low-virulent meningococcal serogroups W-135, 29E, X, Y, Z. 3. Individuals with recurrent MD. Since properdin deficiency probably is the most common deficiency associated with MD it is important that the screening includes the alternative complement pathway.