The DNA content in individual cells from 112 histopathologically normal endometria and from 222 malignant endometrial tumors was measured using flow cytometry. In the normal endometrial cases only single DNA peaks were found, all in the diploid region, and the range of DNA index values was used for defining the limits for diploid tumor cases. In most cases with aneuploid cell populations, an additional peak was found in the normal diploid region. However, combined cytological and histopathological analysis showed that a majority of these diploid cells were to be considered as tumor cells. Aneuploid cell populations were found in 43% of malignant endometria; in the remaining endometrial carcinomas, the flow cytometrical findings showed no differences compared to those of benign tissue. Flow cytometry in this respect did not prove useful as a diagnostic screening method. Ploidy aberrations were correlated to histopathology. Aneuploidy was more common (62%) in poorly differentiated tumors than in highly/moderately differentiated tumors (29%). Two or more aneuploid cell populations were found in 6% of the cases. No difference in aneuploidy was found between FIGO stage I and II (36% and 34%), but aneuploidy was more frequent in stage IV (86%). In normal endometria the fraction of cells with DNA content corresponding to S-phase (S-fraction) was 9.7% on average in the proliferative phase and 6.2% in the secretory phase. In well and moderately differentiated diploid tumors the S-fraction was about the same (8.8% and 9.2%), but in poorly differentiated tumors it was significantly higher (12-16.5%).