From July 1979 to December 1981, 83 patients from 6 centers in the Lombardy underwent radical nephrectomy for category M0 renal cell carcinoma. Postoperatively they were randomly allocated to adjunctive medroxyprogesterone acetate (MPA) or to a control group. After the first year of follow-up, the relapse rate was 13% in the controls (5 of 38 evaluable patients) versus 28% in the MPA treatment group (9 of 32). Sex steroid receptors were studied in both the tumor and in the surrounding healthy parenchyma by the dextran-coated charcoal technique in 52 of 70 clinically evaluable patients. There was no significant correlation between receptor studies and the relapse rate. Besides, receptors were more often detected in the healthy than in the neoplastic tissue, and values rarely exceeded 10 fmol/mg protein. Due to the negative trend in the MPA treatment group, the admittance of patients to this study was stopped after 120 patients had been accepted.