Flow cytometric cell cycle analysis was performed on paraffin-embedded blocks from 49 patients with stage I endometrial carcinoma. Care was taken to separate tumor tissue from normal tissue in each specimen; normal tissue was used as a control for each individual specimen. DNA index, proliferative activity, and cell DNA aneuploidy were correlated with known parameters of tumor malignancy. Increased DNA index corresponded well with the DNA aneuploid tumors, poor tumor differentiation (G3), myometrial invasion of more than one-third, more malignant histologic type of tumor, and low concentration of estrogen (< or = 10 fmole/mg) and progesterone (< or = 25 fmole/mg) receptors. Similar results were obtained for tumor cell proliferative activity (percentage of cells in S + G2/M phases) and for DNA aneuploid tumors. Since more than 90% of patients with stage I endometrial carcinoma survived the 5-year postoperation period, analyzed parameters could not be checked for survival-related prognostic significance. However, our data indicate that cell cycle analysis may be instrumental for objective ranking of several known prognostic parameters.