A series of 73 patients with measurable metastatic disease referred to a tertiary referral centre for consideration for an experimental treatment protocol were entered into a surveillance protocol in order to establish the incidence of spontaneous regression. Initially, patients were taken off the study if metastases showed greater than 25% increase in products of bidimensional measurement but with increasing confidence patients went into therapy protocols only following the development of symptomatic progression. We observed 3 complete and 2 partial unexplained spontaneous regressions and a further 4 patients had prolonged stable disease for more than 12 months. A group of 21 patients on progression subsequently received treatment with alpha-interferon (Wellferon) as part of a multicentre study which included an additional 61 cases; 12 responded (3 surveillance relapse patients and 9 others). Patients with lung metastases only had the highest response rate (10/17 compared with 5 unexplained responses seen in 38 such patients on surveillance). These results confirm that alpha-interferon is active against metastatic renal carcinoma and that the responses are not totally explicable by spontaneous regression.