Inherited deficiency of the complement component properdin is described in a Swedish family without any previous history of meningococcal infections. The properdin-deficient index patient died from a fulminant infection caused by Neisseria meningitidis serogroup Y. Family investigation revealed properdin deficiency in the patient's half-brother and in the maternal grandfather. The half-brother had a history of pneumococcal pneumonia and meningitis probably caused by Borrelia burgdorferi. Opsonic and bactericidal functions of serum were examined in the half-brother after immunization with tetravalent meningococcal vaccine. Vaccination promoted opsonization of N. meningitidis serogroups C and Y but not of serogroups A and W-135. The serum bactericidal activity increased against serogroup C and to some extent against serogroup W-135. This report emphasizes the importance of investigating the complement system even in families with single cases of fulminant meningococcal disease. Individuals with properdin deficiency might be protected from infection by immunization.