In 1243 patients after renal transplantation, 39 malignant tumours were detected in 37 patients. The average latency period between transplantation and tumour disease was 72 months. Tumours included 8 malignant lymphomas, 7 dermatomas and 24 visceral tumours. The patients who developed a tumour had received fewer blood transfusions before transplantation than a tumour-free control group of 60 patients with renal transplants. Rejection crises occurred in a significantly smaller number of tumour patients compared with the control group.