The efficacy and safety of TAT-59 (miproxifene phospate) were compared with tamoxifen citrate (TAM) in ER-positive or ER-unknown patients with advanced or recurrent breast cancer, using the double-blind method. TAT-59 and TAM were both given 20 mg/body orally for over 12 weeks in daily doses. Eligible cases were 93 in the TAT-59 group and 102 patients in the TAM group. The response rate was shown to be 30.1% (28/93) in the TAT-59 group and 26.5% (27/102) in the TAM group. Statistical equivalence between both treatments was proven at 90% confidence interval (-8.5% < or =, < or = 12.8%). Adverse reactions observed in the patients receiving TAT-59 were hot flush, nausea and vomiting, sweating, anorexia, abnormal values in liver function tests, and anemia, showing no differences from the TAM group. Leukopenia (4.9%) and thrombopenia (2.9%) reactions were found only in the TAM group. Two patients in the TAT-59 group and three in the TAM group discontinued treatment due to adverse reactions. However, these adverse reactions were reversible in both groups. In conclusion, TAT-59 was showed comparable efficacy and safety with TAM in ER-positive or ER-unknown patients with advanced or recurrent breast cancer.