SUMMARY.-The effect of the progestogen medroxyprogesterone acetate on metastases from renal, endometrial and other tumours has been studied in 25 patients. Seven patients with renal and endometrial tumours had a useful response, pulmonary metastases and a large primary renal tumour showing the greatest effect. Bony metastases were unaffected by the drug and were treated by local radiotherapy. If a response occurred, it did so within 3 months.IT has been known for several years that the growth of some tumours may be influenced by hormones. Breast tumours will often respond to hormone treatment or to endocrine gland ablation. Briggs et al. (1967) summarised work on the responses of tumours of women, such as those of the breast, endometrium, ovary and cervix, to various progestogens and found an overall response rate of 280%.Renal tumours, which are somewhat more common in males than females, may have endocrine effects, such as polycythaemia and hypertension, due to the release of erythropoietin or renin into the circulation. In the male golden hamster, renal tumours may be produced by chronic oestrogen administration (Matthews et al., 1947;Kirkham and Robbins, 1959), the histological pattern of this tumour being similar to that of human renal tumours (Horning and Whittick, 1954). Both testosterone and progestogens inhibit the production and growth of the tumour; it may be transplanted into the male hamsters which have been oestrogen sensitised, and progestogen may inhibit the growth rate of the transplanted tumour. Cortisone may also have a cancericidal effect, and when given with progestogen in these animals produced almost complete inhibition of tumour growth . Bloom (1964) tried the combination of testosterone and progestogen in 3 patients and corticosteroid treatment in 7 patients with metastatic renal adenocarcinoma without effect, and in another patient progestogen appeared to cause the mastastases to increase whilst testosterone caused them to regress (Bloom and Wallace, 1964). Since then, further trials on patients have taken place. Bloom (1967) reported the effects of steroid treatment in 38 patients (21 men and 17 women) with incurable renal adenocarcinoma and multiple metastases. Of these, 32 had more than one organ involved and 23 were seriously ill or " terminal " when hormone therapy was instituted. In 10 the liver was enlarged, 2 had cerebral metastases, 13 had skeletal deposits and 24 had pulmonary or mediastinal lesions. Six men and 2 women had marked regression of their tumours and