Efficacy and safety of NK 622 (toremifene citrate) were compared with tamoxifen (TAM) by a double blind test in patients with advanced or recurrent breast cancer. NK 622 and TAM were given orally for 12 weeks or more at daily doses of 40 and 20 mg/body, respectively. Eligible cases in NK622 and TAM groups were both 57 patients. No significant difference was observed in patient characteristics between either group. Response rates were 26.3% (8 CR and 7 PR, 15/57) in the NK 622 group and 28.1% (3 CR and 13 PR, 16/57) in the TAM group. Median values of duration to onset of CR were 91 days in the NK 622 group and 169 days in the TAM group. The duration was significantly shorter with the NK 622 group. Median duration of efficacy in CR and PR cases was 155 days in the NK 622 group and 154.5 days in the TAM group. Adverse effects were encountered in 7 patients (12.3%) of each of the 2 groups. The side effects were fatigue, hot flush, WBC decrease, abnormal values in liver function tests, etc. in the NK 622 group and anorexia, nausea, eruption, feeling of warmth, sweating, dry mouth, dizziness, abnormal values in liver function tests, etc. in the TAM group. Administration was discontinued in one patient with eruption and another patient with abnormal values of liver function tests in the TAM group, while there was no such case in the NK 622 group. Including the discontinued cases, the side effects were moderate and reversible in both groups. The patients in whom a drug was determined as useful or more numbered 24/57 (42.1%) in the NK 622 group and 23/57 (40.4%) in the TAM group. There was not significant difference between the 2 groups in the above results except the duration to onset of CR. From these results, NK 622 is expected to show comparable efficacy, safety, and usefulness in patients undergoing TAM treatment for advanced or recurrent breast cancer.