In 35 homosexual men with isolated thrombocytopenia at initial presentation, who were evaluated and treated between 1982 and 1984, hematologic studies showed immune destruction. In contrast to findings in other autoimmune conditions, T-lymphocyte subsets in these patients were reversed, with a mean helper to suppressor ratio of 0.4 and with an absolute depletion of helper cells to 390/mm3. An enzyme-linked immunosorbent assay detected antibodies to human T-lymphotropic virus type III in 21 of 25 patients tested; Western blot analysis confirmed seropositivity in the other 4 patients, who had borderline findings. Although 19 of 24 patients treated with steroids responded, only 2 achieved sustained normal platelet counts. Ten of fifteen patients who had splenectomy achieved remissions. Three patients treated with steroids or splenectomy developed diagnoses compatible with the acquired immunodeficiency syndrome 16 to 34 months after their initial presentation with thrombocytopenia. These findings indicate that immune thrombocytopenia is part of the clinical spectrum of the acquired immunodeficiency syndrome.