Cancer may be serious late effect of marrow transplantation. Radiation, chemotherapy, immunosuppression and the original disease for which transplantation was performed may predispose to the development of cancer. 116 of 9732 patients reported to the IBMTR (International Bone Marrow Transplant Registry) have developed a new malignancy. Late effects were evaluated by the EBMT-EULEP (European Bone Marrow Transplant-European Late Effect Project) Late Effect Study Group in 147 patients surviving 6 years and 79 patients surviving more than 10 years. New malignancies developed in 11 of these patients. Lymphomas and leukemia comprised 73 cases reported to the IBMTR and one case reported to the EBMT-EULEP study. Tumors of the skin, oropharynx, vulva vagina and cervix prevailed in 41 patients with solid tumors. The distribution of malignancies is similar to that observed in organ transplant patients not given radiation or chemotherapy and suggests immunosuppression as a major contributory factor. In dogs the incidence of malignancies was studied after either chemotherapy or total body radiation in various regimens and marrow transplantation. Both chemotherapy and radiation shortened tumor-free survival in comparison to untreated dogs. Higher doses, larger fractions and shorter treatment schedules enhanced earlier tumor development. Soft tissue sarcomas and thyroid carcinoma were most frequent in treated, mammary carcinoma in untreated dogs. In treated dogs deaths from cancer were observed starting at the age of 5 years as compared to untreated dogs at the age of 9 years. The data from animal experiments indicate that the incidence of solid tumors in marrow transplant patients may still rise in the coming decades.