The DNA content of the tumour cells in 10 patients with primary renal cell carcinomas was analysed; from six of the patients skeletal metastases were also studied. Four patients had homogenously diploid primary tumours, with solitary metastases. Six patients had aneuploid primary tumours, three with solitary and three with multiple metastases. In two patients radical excision of diploid metastases resulted in long disease-free intervals. Patients with diploid tumours survived significantly longer than patients with aneuploid tumours. These results indicate that tumour DNA content might be a useful prognostic indicator. The measurement of DNA content may be a suitable method of identifying those patients likely to survive long enough to benefit from major surgical resection and reconstruction.