Within a period of six months, a 20-year-old female with a homozygous deficiency of the C5 component of complement developed meningococcal meningitis twice (different serogroups). Additional C5 deficiencies were not found in relatives. Homozygous deficiency of C5 was also present in another family in which a 16-year-old female and an 18-year-old sister suffered from meningococcal meningitis. Some characteristics of meningococcal disease in patients with C5 deficiency differed from meningococcal disease in patients with a normal complement system: meningitis occurred at a relatively advanced age, was associated with serogroups W-135, B and X and recurred in two of three patients.