The DNA content in renal cell carcinoma as an indicator of prognosis was studied retrospectively in 55 patients without distant metastases at operation. Two groups of patients were selected, differing with respect to survival. Thirty-three patients survived for at least 10 years and 22 succumbed to the disease within four years. DNA measurements in morphologically identified tumor cells were performed in histological sections by single cell cytophotometry. The tumor cells of the surviving patients had a DNA content comparable to that of normal cells. A diploid/near diploid DNA content was the dominant feature in 32 of these 33 tumors. The remaining patient had a tumor with a tetraploid/near tetraploid DNA value. In contrast, all tumors from the non-surviving patients had abnormally increased DNA content, indicating a high degree of aneuploidy in these tumors. The results suggest that DNA content may be superior to other clinical and microscopical parameters as a prognostic indicator.