Toremifene is a triphenylethylene derivative structurally and pharmacologically similar to tamoxifen. This Phase I trial assessed the safety, pharmacokinetics, anti-estrogenic, and estrogenic effects of toremifene at six dose levels (10, 20, 40, 60, 200, and 400 mg/day). The most common side-effects associated with therapy included gastrointestinal (nausea/vomiting 43%), anti-estrogenic (hot flashes 29%), and CNS (dizziness/vertigo 12%). Three patients with bone metastases from breast cancer developed hypercalcemia. At doses greater than or equal to 40 mg/day a decline in LH and FSH occurred which was not statistically significant. At all doses tested SHBG rose during therapy. A dose dependent estrogenic blockade was seen on the vaginal epithelium following challenge with transdermal estradiol. Steady-state concentrations of toremifene were reached within 4 weeks, and at doses greater than or equal to 60 mg/day ranged from 879-3445 ng/ml. The half-life was found to be 5 days, and at three weeks following discontinuation of treatment concentrations greater than 24 ng/ml were detected. The N-desmethyl and 4-hydroxy metabolites achieved steady state levels within 4 weeks and had half-lives of 6 and 5 days respectively. Partial responses were seen in 4 patients, 3 with breast cancer treated at 200 mg/day and 1 with endometrial cancer treated at 400 mg/day.