In 70 renal cell carcinomas, nuclear DNA content was determined by means of flow cytometry (FCM) and image cytometry (ICM). The two methods produced comparable results as to DNA tumor ploidy (DNA tumor stemlines, DNA index): 14 of the tumors were tetraploid or aneuploid and 56 diploid. Results with the two methods were also comparable in a comparison of DNA ploidy with degree of tumor malignancy (tumor grade G1-3) and local tumor spread stage (pT stage). As a consequence, both methods appear suitable as means of determining DNA tumor ploidy and thus of formulating a prognosis in renal cell carcinoma. Renal cell carcinomas with diploid stemlines tend to be characterized by local growth, whereas tetraploid or aneuploid tumors show a tendency toward perirenal spread and venous invasion.