Although only 10-20% of all cases of marked atypical hyperplasia (MAH) progress to cancer, usually all patients with this diagnosis are treated with hysterectomy. The lack of diagnostic criteria that can accurately predict the outcome of the disease is the major cause for this overtreatment. Recently, a classification rule consisting of nuclear morphometric features has been described, which can predict the outcome in the majority of these patients. In the present study, this predictive classification rule is tested in 42 cases of MAH, obtained from a total of 2,662 curettements diagnosed as hyperplasia or carcinoma. Among these 42 cases, eight (17%) progressed to cancer. In the classification rule, the following decision scheme is used: Classification value F = -7.13 + 1.24 X (mean maximal nuclear diameter) -3.00 X (SD maximal nuclear diameter), where F less than 0 means: no progression, F greater than 0 means: progression. Of the eight progressive cases, seven scored above zero and one just below that value (i.e., F = -0.3). Although a considerable number of the nonprogressive cases had values above zero, 11 (32%) scored below zero. Thus, in this independent "test set" material the nuclear morphometric classification rule proved to be accurate and useful in predicting the outcome of patients with MAH of the endometrium. Women with a score below -0.5 can be considered for a more conservative ("wait and see" or hormone treatment) therapeutic approach. This can be especially important in young patients.