Background: Atrophic epithelium of cervix sampled from postmenopausal women may mimic high-grade cervical intraepithelial neoplasia in Papanicolaou-stained (Pap) smears. Ki-67 (MIB-1) protein presents on proliferating cells, and percentage of cells with positive nuclei provides a reliable tool for rapid evaluation of the growth fraction. The aim of this study was to determine the diagnostic value of protein Ki67 staining in atypical pap smears of postmenopausal women. Methods: In a case-control setting, pap smears of 75 women with an atypical pap smear (case group) and 75 with normal pap smears (controls) were obtained before and after estrogen treatment. Afterward, samples were exposed to the monoclonal antibody Ki-67 (MIB-1) and the immunohistochemically demonstrated Ki-67+ cells were compared. Results: Mean ages of cases and controls were $60.4{pm}4.5$ and $59.9{pm}4.3$ years respectively (P=0.50). There was one (2.7%) positive Ki-67 specimen in the case group, without any positive Ki-67 specimen in the control group (P=0.50). Conclusions: Measurement of proliferative activity index in Pap smears restrained with MIB1 is a simple, reliable, and cost-effective method for excluding negatives. This would imply that it might allow a substantial reduction of diagnostic estrogen courses and subsequent Pap smears in postmenopausal women with atypical findings.