Ninety-nine (21%) of 471 patients who survived with functioning grafts for at least six months following renal transplantation developed cancer. Of these 76 (77%) had skin malignancy, 29 (29%) had malignancy affecting other organs, and six had cancer of both skin and other organs. In patients with skin cancer squamous cell carcinoma (SCC) was three times as frequent as basal cell carcinoma (BCC). SCC tended to be multiple, recurrent and aggressive. Seven (12%) patients with SCC developed metastases of whom five died. Cancers other than skin included reticulum cell sarcoma (9), acute leukaemia (2) and cancers involving the gastrointestinal (5), genitourinary (11) and respiratory (2) systems. Incidence of cancer in patients surviving beyond one, five and nine years after operation was 98/428 (23%), 70/179 (39%) and 20/45 (44%) respectively. In 31 patients who died more than five years after transplantation cancer was the major cause in eight (26%). For the types of cancers recorded estimates show allograft recipients to be at increased risk when compared with the age-matched Australian population by factors which varied from 300 times for reticulum cell sarcoma to 1.8 times for invasive carcinoma of the cervix. The full extent of the threat of cancer in immune suppressed transplant recipients remains to be determined.