The frequency of C4A gene deletion was studied in Japanese patients with systemic lupus erythematosus (SLE) and was compared with healthy controls. DNA preparations were extracted from peripheral blood leukocytes from 59 patients with SLE and from 166 healthy persons, and digested by restriction enzymes. They were hybridized with C4 complementary DNA by the Southern blotting method and the deletion of C4A gene was judged from restriction fragment length polymorphism. At the same time phenotypic C4A deficiency (C4AQ0) was measured. Our results showed that the frequency of phenotypic C4A deficiency was 44.1% in Japanese patients with SLE and this value was comparable with that (43.2%) in Caucasian patients. On the other hand the deletion of C4A gene was not found in Japanese patients with SLE (0%), or in healthy controls (0.6%). Our results indicate that C4AQ0 may contribute to the pathogenesis of SLE beyond the ethnical differences but Japanese patients with SLE have a different genetic background from Caucasian patients with the C4A gene deleted.