We describe an 11-year-old girl suffering from recurrent meningitis with a complete absence of the seventh component of complement (C7). Diagnosis was established by haemolytic titration and western blotting. The patient's serum lacked the 85 kDa C7 chain. Haemolytic activity of serum was reconstituted with either pooled normal human serum or with purified C7. The relatives (parents and one sister) had half-normal levels of both immunochemically and functionally determined C7, indicating a heterozygous state for C7 deficiency.