Danazol, an attenuated androgen, has recently been introduced into the treatment of autoimmune thrombocytopenia. We studied its effects on T helper/inducer (Thi) and T suppressor/cytotoxic (Tsc) lymphocytes in these patients. Prospectively nine patients were studied with their T-cell subsets measured before and during danazol therapy. Increases in the percentage of Thi lymphocytes (P less than 0.05) and Thi/Tsc ratios (P less than 0.001) were observed at 1 and 3 months of treatment. Retrospectively T-cell subset data on 30 patients not treated with danazol and 36 patients on danazol were compared with those of 35 normal controls. The group not on danazol had lower percentages of Pan T (P less than 0.05), Thi (P less than 0.002), and Thi/Tsc ratios (P less than 0.00005), and had higher percentages of Tsc lymphocytes (P less than 0.01), than those of controls. In the group treated with danazol the percentages of Pan T, Thi, and Tsc lymphocytes were similar to those of controls. The percentage of Thi in the treated group was higher (P less than 0.002) than in the untreated group. Thus, danazol appears to be an effective immune modulator, correcting the abnormality of T-cell subsets seen in autoimmune thrombocytopenia by increasing the percentage of Thi lymphocytes.