Fifty-four (54) unrelated patients with Mediterranean Kaposi's sarcoma (MKS) and 8 patients members of 4 unrelated families with familial MKS were serotyped for HLA-A,B and DR antigens. The diagnosis was histologically confirmed and all patients were negative for anti-HIV antibodies. An increased frequency of HLA-B18 (44.4% vs 14.2% in the controls, p < 0.001, RR = 4.8) and HLA-DR5 (57.6% vs 37.2% in the controls, p < 0.025, RR = 2.29) was observed in the group of patients with MKS. Seven (7) of the 8 family members with FMKS possessed HLA-DR5, and the affected members in the 3 families shared a common haplotype which included HLA-DR5. These findings support the hypothesis that genetic factors linked to HLA-DR5 antigen may contribute to the pathogenesis of MKS.