From July 1, 1979 to June 30, 1983, 136 consecutive patients from 5 centers in Lombardy entered a prospective randomized study to compare 500 mg. adjuvant medroxyprogesterone acetate 3 times a week for 1 year to no treatment following radical nephrectomy for category M0 renal cancer. After a median followup of 5 years (range 42 to 90 months) 40 of 120 evaluable patients (33.3 per cent) experienced relapse after a median interval free of disease of 17 months (range 2 to 74 months). Relapses occurred in 19 of 58 evaluable patients in the adjuvant treatment group (32.7 per cent) and in 21 of the 62 evaluable controls (33.9 per cent). Sex steroid hormone receptors were studied in 102 of the 120 evaluable patients with the dextran-coated charcoal technique. No significant correlation could be found among receptors, relapses and treatment. On the other hand, 33 (56.9 per cent) of the 58 treated patients experienced 39 complications related to the long-term hormonal therapy. Three patients had to discontinue medroxyprogesterone acetate for severe toxicity after 2 to 3 months. Medroxyprogesterone acetate cannot be recommended as adjuvant therapy to radical nephrectomy in patients with renal cell carcinoma.