Ploidy and cell-cycle stage were determined by flow cytometry (FCM) in 46 human renal carcinomas. Cell populations with aneuploid DNA were detected in 46% of these. In the investigated samples, the fraction of cells with abnormal DNA content varied from 8 to 100%. The proliferative activity was generally low as indicated by the small fractions of cells in S and (G2 + M) phases. This was confirmed by the labelling indices on autoradiographic slides. The fraction of cells in phases S and (G2 + M) for tumours that were pre-irradiated with 15 or 25 Gy before nephrectomy was only slightly less than in unirradiated tumours. Comparison of the FCM ploidy with the results of histological grading showed that all cases classified as the most malignant grades IV or IIIB (according to the nuclear and to the combined grading system of Syrjänen and Hjelt (1978) were hyperdiploid. On the other hand, 45% of the hyperdiploid and 89% of the diploid tumours were of the low grades I and II. After a follow-up for 6 months to 2 years, 8/17 patients with hyperdiploid and only 1/14 patients with diploid tumours have died or relapsed with multiple metastases. The results indicate that the aneuploidy of tumours, measured by FCM, might provide useful additional information for prognosis.