High dose medroxyprogesterone treatment was given to 20 patients with metastatic renal cell carcinoma. Distant metastases occurred before the perifascial nephrectomy in 11 patients and following nephrectomy in 9. Tumor deoxyribonucleic acid content was analyzed by flow cytometry in 8 fresh samples from each primary tumor. Four patients had homogeneously diploid primary tumors, 5 had tumors with diploid and aneuploid samples, and all 8 tumor samples were aneuploid in 10 patients. Deoxyribonucleic acid analysis was unsuccessful in 1 patient. One patient with a diploid primary tumor died of an intercurrent disease. Three patients (16 per cent) had objective remissions and 1 had a long-lasting stable disease. Of the 4 patients with any response to medroxyprogesterone acetate treatment 3 had diploid primary tumors, and 1 had 8 diploid and 2 aneuploid samples in the primary tumor. The remaining 14 patients showed no response to treatment and had progressive disease (11 of these patients died within 14 months). All 14 patients had aneuploid primary tumors. The results indicate that tumor ploidy might be related to response to medroxyprogesterone acetate treatment. Deoxyribonucleic acid content seems to be an important parameter to consider in planning treatment of metastatic renal cell carcinoma.