Anticardiolipin antibody levels were determined in 73 homosexual men. Thirty of these patients had acquired immunodeficiency syndrome (AIDS), 16 patients had the AIDS-related complex (ARC) and 27 were healthy. Antibodies to human T-lymphotropic virus type III (HTLV-III) were detected in all patients with AIDS and ARC and in 11 of the healthy homosexuals. Eight patients with positive fluorescent treponemal antibody absorption test were excluded from the study. High levels of IgG-anticardiolipin antibodies were present in 23 of the 28 patients with AIDS; 12 of the 14 with ARC; five of the 10 HTLV-III positive healthy homosexuals; and none of the 13 HTLV-III negative healthy homosexuals. High levels of IgM-ACA were detected only in four patients with AIDS. The IgG-anticardiolipin levels were higher in the HTLV-III positive patients than in the HTLV negative group (P = 0.013). None of the patients with anticardiolipin antibodies exhibited venous thrombosis.