We investigated the association of CYP17 gene polymorphism with the risk of having endometrial cancer and a well-known precursor of it, endometrial hyperplasia. Group A (control group) consisted of 35 patients who had histologically proven normal endometrium. Group B and C consisted of 18 and 30 patients who had endometrial hyperplasia with and without atypia, respectively. Group D consisted of 57 patients who had endometrial cancer. Venous blood samples were collected from patients in groups, and polymerase chain reaction was performed to determine the CYP17 gene polymorphism. Significant increase of A1/A1 and a decrease of A1/A2 genotype frequencies have been determined in patients with endometrial cancer and with atypical endometrial hyperplasia. No significant differences were found between groups in the frequency of A2/A2 genotype. There was no significant difference between the groups in the meaning of allele distributions. CYP17 polymorphism had correlation with endometrial atypia and cancer. Related effects of different types of CYP17 gene variants on the progression of hyperplastic endometrial cells into carcinoma should be evaluated in further studies. Progress in this area would help us modulate preventive treatments used in those actual high-risk group patients.