The frequencies of HLA-A, B, C, DR and DO antigens were studied in 63 Japanese patients with subacute sclerosing panencephalitis (SSPE). The results could not confirm the statistical association between SSPE and HLA antigens. In addition, the allele frequencies of complement components C4, C2 and BF were determined in the 20 haplotypes with 10 unrelated SSPE patients. A significant association of C4A'QO with SSPE in Caucasians was not found in Japanese.