We performed phase I study of FK 435, a new antiestrogen, in 30 patients with advanced breast cancer. Slight to moderate adverse reactions were noted as follows. Single-dose study: anorexia, nausea, lassitude in one patient (80 mg), decreased serum calcium in one (160 mg), redness, tenderness in one, facial flushing, hot flushes, headache in one (320 mg). Repeated-dose study: anorexia, nausea in one patient (40 mg/day), anorexia, diarrhea, increased FSH in one, increased PRL in one (80 mg/day). FK 435 was well tolerated. Tmax was 3-5 hours, T1/2 about 25 hours. Most of FK 435 was excreted into urine as glucuronide.