Since November 1982 we have seen an association of thrombocytopenic purpura with chronic narcotic addiction in 70 patients with a mean platelet count of 53 000 ±4000 (SE); 33 had stopped taking intravenous drugs for an average of 21.2 ±4.7 months; 13 of 15 had elevated antibody titers for a virus related to the acquired immunodeficiency syndrome. Platelet-bound IgG, IgM and complement levels were 16.7-, 5.6-, and 3.1-fold greater than control values, respectively, and 2.6-, 1.9-, and 2.4-fold greater than values in 25 patients with classic autoimmune thrombocytopenic purpura studied at the same time. Thirty-three of thirty-six addicts had elevated circulating immune complexes, whereas 8 patients with autoimmune thrombocytopenia had no elevation. Eleven of eighteen addicts had positive serum platelet-reactive IgG titers, compared to 5 of 19 patients with classic autoimmune thrombocytopenia. The platelet-reactive IgG in sera of addicts was composed of 7S IgG antibody as well as high molecular weight (immune complex) IgG. Thus, chronic addicts appear to have a new immunologic platelet disorder associated with the presence of 7S IgG antiplatelet antibody, like patients with classic autoimmune thrombocytopenic purpura, and immune complex associated nonspecific platelet IgG, like male homosexual patients with thrombocytopenia.