A monoclonal antibody, MSN-1, was used for flow cytometric analysis of cells from normal endometrium, endometrial hyperplasia, and endometrial carcinoma. The 90th percentile of the specimen control was used as a threshold. Reactivity was defined by the percentage of stained cell about the thresholds, adjusted for the expected percentage. A sample was considered positive if the reactivity exceeded 10%. The positivity rate for normal endometrium, endometrial hyperplasia, and endometrial carcinoma specimens was 9.2%, 19.2%, and 84.6%, respectively. The mean (+/-SD) reactivity rate of MSN-1 for normal endometrium, endometrial hyperplasia, and endometrial carcinoma was 3.3 +/- 6.2%, 7.4 +/- 13.8%, and 34.4 +/- 24.2%, respectively. There was a significant difference of the reactivity rates between normal endometrium and endometrial hyperplasia, and between endometrial hyperplasia and endometrial carcinoma (P < 0.01). In the subgroup of endometrial carcinoma patients with confined muscular invasion, the reactivity and the positivity rates were also analyzed. There was no relationship between the depth of muscular invasion and the reactivity rate or the positivity rate. In endometrial carcinoma patients who simultaneously underwent endometrial cytology, the relationship of the results of cytology and the positivity rate was also analyzed. When the results of cytology and flow-cytometric analysis were combined, the positivity rate of endometrial carcinoma was 100% (30/30). In conclusion, flow-cytometric analysis of endometrial cells employing MSN-1 could be a useful supplementary diagnostic method for endometrial carcinoma.