The increased risk of cancer in patients who have had kidney transplants has mainly been attributed to immunosuppressive therapy; however, the prior period of uraemia and dialysis has also been postulated as a cofactor. We analysed cancer risk retrospectively in a cohort of 4178 patients undergoing renal replacement therapy, of whom 3592 were treated with dialysis alone and 1821 later had transplants. We found that excess cancer risk in such patients occurred after transplantation and not during dialysis.