A role has been suggested for 1 alpha,25-dihydroxyvitamin D3 (1,25-(OH)2D3) in cell proliferation and differentiation. Therefore the concentration of the 1,25-(OH)2D3 receptors and DNA-ploidy was measured in 22 renal cell carcinomas. No relation was found, the mean 1,25-(OH)2D3 receptor concentration being 8.4 fmol/mg of protein (range 2.8-15.9) in diploid tumors and 7.0 fmol/mg of protein (range 0-27.8) in DNA aneuploid tumors. The aneuploid tumors (11 out of 22) had a heterogeneous DNA content in 9 out of 11 cases. At the time of operation, no patient had metastases. In this prospective study, one out of 9 patients with DNA aneuploid tumor had died of renal cell carcinoma (observed 33-58 months, median 42 months). None of 10 patients with diploid tumors had died (observed 40-56 months, median 52 months).