Some 27 (5·5 per cent) of 492 renal transplant recipients developed de novo cancer between January 1975 and December 1991. Patients administered triple therapy of prednisolone, cyclosporin A and azathioprine had a significantly higher incidence of cancer (seven of 40 patients; 17·5 per cent) than those given prednisolone with cyclosporin (14 of 319; 4·4 per cent) and azathioprine with prednisolone (six of 133; 4·5 per cent) (P= 0·005). In a prospective study between January 1989 and December 1992, 110 renal transplant patients were randomized into three immunosuppressive regimens at the time of transplantation. The incidence of cancer in patients receiving low-dose cyclosporin, azathioprine and prednisolone was three of 45, in those given high-dose cyclosporin and prednisolone none of 23 and in those administered high-dose cyclosporin, nifedipine and prednisolone one of 29. The addition of azathioprine to ongoing maintenance cyclosporin and prednisolone therapy is useful in a subgroup of patients with graft dysfunction, but there are possibly higher risks in the development of de novo carcinoma.