The aim of current study was to evaluate the diagnostic as well as the prognostic significance of the mitotic index (MI) in endometrial adenocarcinoma. We compared the MI in normal endometrium, endometrial hyperplasia, and endometrial adenocarcinoma. The mean MI in normal proliferative endometrium (4.35 +/- 3.4) was not significantly different from those in glandular hyperplasia (4.19 +/- 6.0) and well-differentiated adenocarcinoma (4.01 +/- 4.2). A significantly higher MI (10.7 +/- 8.2) was found only in poorly differentiated adenocarcinoma (P less than 0.05). Results of our work indicate that the MI cannot be used as a discriminating factor in the differential diagnosis of borderline cases of endometrial hyperplasia and endometrial adenocarcinoma. We examined the usefulness of the MI, grade of differentiation, and depth of invasion as the prognostic factors in endometrial adenocarcinoma. The significantly higher 5-year mortality rate was associated with an MI greater than 5, grade III of differentiation, and neoplastic invasion penetrating to the outer third of the myometrium. All of the patients with MI greater than 5 had tumors with the highest grade of differentiation and/or invasion involving the mid and outer third of myometrium. This suggests that the higher mortality of patients with an MI greater than 5 reflects the presence of anaplastic and/or highly invasive tumor.