The incidence of cancer in 646 dialysis/transplant patients before uraemia developed, during the period of progressive uraemia, and post-transplantation was compared. 10 tumours (3 breast, 2 kidney, 1 leukaemia, 1 lung, 1 insulinoma, 1 thyroid, 1 cervix in situ) developed in 9 patients during the period of progressive uraemia, a significant increase over the expected number in the age-matched general population. 6 of these patients have received transplants and have no evidence of recurrent disease 6 months to 4 years post-transplantation. 11 de-novo tumours have developed in 530 transplant recipients (4 cervix in situ, 2 skin, 2 reticulum-cell sarcoma, 1 lip, 1 dysgerminoma, 1 colon)--a significant increase over the age-matched general population. The cancers in the uraemic patients are relatively common types of mesenchymal tumours while the cancers in the transplant recipients are epithelial and lymphoproliferative. This difference may reflect the presence of the graft in the transplant patient or may be due to different patterns of immunosuppression in these two populations.