Aminoglutethimide (AG) was administered as palliative therapy in 112 patients with metastatic breast cancer. In 36 patients, the dose level was 1000 mg/day; 76 patients received a dose level of 500 mg/day. Patients with brain or liver metastasis were excluded, as were patients with tumors determined to be negative for estrogen receptors. Objective regression was observed in 35 (31%) patients, with the duration of response ranging from 4 to 36 + months (mean, 12 months; median, 10 months). Response was observed in 11 of 31 (35%) patients with soft tissue metastasis; 16/59 (27%) patients with osseous metastasis; and 8 of 22 (36%) having visceral metastasis. In 93 patients with positive estrogen receptor (ER), 33 responded (35%), whereas in 19 patients with unknown ER status, two responded (11%). Response to previous treatment with tamoxifen (TAM) had occurred in 31 patients; of these, response to AG was noted in 11 (35%). Of 24 patients failing to respond to prior treatment with tamoxifen, four (17%) responded to subsequent therapy with AG. Thirteen patients had previously received combination chemotherapy, and response to AG was noted in two (15%). The side effects observed in this study included skin rash in ten patients, fever in eight, somnolence in three, weakness and dizziness in one, headache in one, insomnia in one, dyspnea in one, and ataxia in one. Treatment had to be discontinued in eight patients, due to the severity of the side effects. As expected, patients receiving AG at the lower dose level of 500 mg/day experienced fewer and less severe side effects than those treated with the higher dose. The response rate in the 1000 mg/day group was 10/36 (28%) and in the 500 mg/day group, it was 25/76 (33%). The lower dosage was better tolerated without apparent compromise in therapeutic efficacy.