HLA-A, B, DR phenotype frequencies were studied in 69 homosexual patients with persistent generalized lymphadenopathy (PGL), in 8 patients with LAV/HTLV III related thrombocytopenic purpura and in 21 homosexual controls. Antigen DR5 was significantly increased in PGL patient groups as compared to controls. Our findings suggest that genetic factors associated with HLA-DR5 antigen may predispose individuals infected by the LAV/HTLV III virus to the occurrence of PGL and/or thrombocytopenia.