We evaluated the complement system in 20 patients presenting with a first episode of meningococcal meningitis, meningococcemia, or meningococcal pericarditis. Assays of total serum complement activity were performed prospectively in 12 patients and retrospectively in 8. Six of the twenty patients had a complement deficiency (CH50 greater than 2 S.D. below the normal mean). Three of these six had a deficiency of a terminal-pathway protein (C6 in two and C8 in one), and the other three had deficiencies of multiple complement proteins associated with underlying systemic lupus erythematosus or multiple myeloma. Patients with decreased amounts of complement were similar to normal patients in terms of sex, age, type of infection, and meningococcal serogroup, but 3 of the 6 patients with a complement deficiency were black, as compared with none of the 14 patients with normal function (P = 0.018). Complement deficiency is common in patients with a first episode of meningococcal disease and may be due to either a deficiency in a single terminal protein or a complement-depleting underlying illness.