PURPOSE:To evaluate the effects of letrozole (Ltz) in carcinogen+estrogen-induced endometrial hyperplasia. METHODS:BALB/c female mice were divided into four groups of 12 animals each receiving an intrauterine dose of N-ethyl-Nnitrosourea (ENU) and weekly subcutaneous injections of estradiol hexaidrobenzoate (EHB), except for group I(control). The groups were divided in I (control), II (ENU+EHB), III (ENU+EHB+MPA) and IV (ENU+EHB+Ltz). Group III also received intramuscular injections of MPA (medroxy progesterone acetate) every four weeks, while group IV received oral doses of Ltz daily. At the end of 16 weeks, the animals were sacrificed, and blood samples were collected for the measurement of serum estradiol and progesterone levels.Uterine histological sections were made to evaluate the presence of endometrial proliferative lesions. Differences between groups were evaluated with student's t test, ANOVA and chi-square test. RESULTS:Groups ENU+EHB, ENU+EHB+MPA and ENU+EHB+Ltz showed varying degrees of endometrial hyperplasia. The incidence of hyperplasia in groups ENU+EHB and ENU+EHB+Ltz was higher and more severe than in group ENU+EHB+MPA.Control group showed lower levels of serum estradiol than the other groups. CONCLUSION:There was no evidence that letrozole could act as an antiestrogenic drug in the development of endometrial proliferative lesions.