Background. After transplantation, cancer risk varies from no increase for several common cancers to a many-fold increase for a number of, chiefly virus-associated, cancers. The smaller excess of cancer in dialysis has been less well described, but two studies suggested that impaired immunity might be responsible. Methods. In a population-based cohort study of 28 855 patients who received renal replacement therapy (RRT), we categorized incident cancers as end-stage kidney disease (ESKD) related, immune deficiency related, not related to immune deficiency, or of uncertain status, according to whether they were, or were not, increased in published reports of cancer in ESKD prior to starting RRT, organ transplantation or human immunodeficiency virus infection. Standardized incidence ratios for, and excess burdens of, cancer were calculated for all persons normally resident in Australia starting treatment by dialysis or renal transplantation from 1982 to 2003. Results. The risk for ESKD-related cancers was increased 4-fold in dialysis and during transplant function. For immune deficiency-related cancers, the increase was 1.5 (95% CI 1.3–1.6) times in dialysis, and 5-fold after transplantation. ESKD- or immune deficiency-related cancers contributed to ~90% of the excess burden of cancer, 48% and 36%, respectively, in dialysis, and 10% and 78% after transplantation. The remaining excess malignancy was contributed by cancers whose relationship with ESKD and immune deficiency is not yet certain. Conclusions. In RRT, the increase in cancer is restricted, largely if not wholly, to cancers with origins in ESKD or related to immune deficiency. For the former, the cancer risk is similar in dialysis and transplantation, but for immune deficiency-related cancers, the relative risk is much greater after transplantation. ; John H. Stewart, Claire M. Vajdic, Marina T. van Leeuwen, Janaki Amin, Angela C. Webster, Jeremy R. Chapman, Stephen P. McDonald, Andrew E. Grulich and Margaret R. E. McCredie