Intrinsic estrogenicities of the selective estrogen receptor modulators (SERMs) toremifene 60 mg daily or 200 mg daily and tamoxifen 20 mg daily (TOR60, TOR200 and TAM20) were compared in a randomized clinical study in postmenopausal women with advanced breast cancer. The study was open label in three parallel groups. Variables for analysis were serum follicle stimulating hormone (FSH), luteinizing hormone (LH), sex hormone binding globulin (SHBG), estradiol (E2), antithrombin III (AT III), aspartate aminotransferase (ASAT) and vaginal cytology. Clinical efficacy and safety have been reported earlier. A total of 648 patients were randomized (221 to TOR60, 212 to TOR200 and 215 to TAM20). Sera were available for the analysis from 148, 165 and 156 and for vaginal cytology from 98, 93 and 86 patients, respectively. All treatment regimens showed tissue-specific and dose-dependent estrogen agonist effect. In the primary measure of in vivo estrogenicity, effect on hypothalamus-pituitary-axis, all three treatment regimens decreased serum FSH (p < 0.001). TOR200 was more potent than the two other treatments (p < 0.05), but surprisingly, TAM20 was more estrogenic than TOR60 (p < 0.001). As could be expected in postmenopausal women, the treatments had no effect on mean serum E2 concentrations and decrease of serum LH was similar to that of FSH. Estrogenic effect on the liver was seen as dose-dependent increase of SHBG with statistically significant differences between the treatment groups (p < 0.001). Trends of transient ASAT elevations in TOR200 group (p = 0.07) and in all treatment groups AT III decrease (p = 0.1) were seen in the beginning of the treatment. TOR60 or TAM20 did not have an effect on mean ASAT values, and AT III decreased in TAM20 group more than in the two other groups (p = 0.1 compared to TOR60 and p < 0.05 compared to TOR200). Estrogenic effects on vaginal superficial cells were higher in TOR60 and TOR200 groups when compared to TAM20 (p < 0.05). Toremifene and tamoxifen had tissue-specific and partially dose-dependent estrogenic effects in hypothalamus-pituitary-axis, in the liver and in the vaginal epithelium of postmenopausal women. In some tissues tamoxifen 20 may be more estrogenic than toremifene 60 mg/day.