A 13-year-old boy presented with juvenile chronic arthritis of 6 months' duration. Antinuclear antibodies, anti-double-stranded DNA antibodies, and rheumatoid factor were not detected. Western blotting showed a deficiency of the beta subunit of the eighth component of complement. The same deficiency was present in the patient's sister. C8 beta deficiency is usually detected in individuals who survive meningococcal disease. There was no such history in this family. Juvenile chronic arthritis has not previously been described in patients with C8 beta deficiency.