Almost all complement component 9 (C9) deficiency in Japan shows Arg95 Stop mutation of C9 gene. Therefore, we studied the prevalence of Arg95Stop mutation of C9 gene among 78 patients with SLE to elucidate the association of SLE and C9 deficiency. The Arg95Stop carrier frequency showed no significant difference between SLE patients and controls. Thus, C9 deficiency is not implicated in SLE susceptibility.