Nuclear parameters from 24 cases of atypical endometrial hyperplasia were determined by means of graphic tablet and microcomputer. Eight of the 24 hyperplasias progressed to carcinoma, but the remaining 16 did not progress during a mean follow-up period of 11.8 years. A linear discriminant function selected the mean and standard deviation of maximal nuclear diameter as useful predictors of clinical outcome. The linear discriminant function predicted the correct outcome in 83% of the cases. This study suggests that measurement of nuclear parameters in atypical endometrial hyperplasia may provide a more objective means of predicting the behavior of the various forms of hyperplasia.