To assess whether a higher cumulative tamoxifen dose is associated with increased incidence of various types of endometrial pathologies, we compared cumulative dose of tamoxifen treatment as well as demographic characteristics, risk factors for endometrial cancer, transvaginal ultrasonographic endometrial thickness, and various treatments for the primary breast cancer between 159 postmenopausal breast cancer tamoxifen-treated patients without endometrial pathologies (group I) and 67 similar patients with endometrial pathologies (group II). A similar comparison was made between group I patients and similar patients with proliferative endometrium (group IIa), with endometrial hyperplasia (group IIb), with endometrial polyps (group IIc), and with endometrial cancer (group IId). Overall cumulative tamoxifen dose was significantly higher in group II as compared to group I (27.4+/-33.4 and 17.4+/-20.2, respectively; P<0.0252). Transvaginal ultrasonographic endometrial thickness was significantly higher in group II than in group I patients (16.3+/-11.3 mm and 12.1+/-6.3 mm, respectively; P<0.0147). The frequency of diabetes mellitus, of previous postmenopausal bleeding, and of previous exposure to hormone replacement therapy was significantly higher in group II than in group I patients (P<0.001, P<0.0001 and P<0.001, respectively). There were no significant differences in all parameters tested between group I, group IIa, group IIb, group IIc, and group IId. However, there was an obvious trend for higher cumulative tamoxifen dose in patients with benign endometrial pathologies as compared to those without endometrial pathologies or to those with endometrial cancer (Group I = 17.4+/-20.2 g, group IIa = 22.5+/-18.5 g, group IIb = 28.1+/-20.3 g, group IIc = 31.4+/-42.7 g and group IId = 10.4+/-12.6 g). Endometrial pathologies, except for endometrial cancer, are associated with a high cumulative dose of tamoxifen in postmenopausal breast cancer patients.