The incidence, predisposing factors, localization, evolution and outcome of neoplasms following kidney transplantation were studied in two groups of patients--120 and 146 patients for an observation period from 1 to 16 years. In patients with adequate renal function who received immunosuppressive treatment for more than one year (accordingly 78 and 88 patients) neoplasms developed in 4 and 10 patients with mean duration of immunosuppressive treatment 4.9 and 6.1 years respectively. The neoplasms were: 3 skin cancers, 2 lung cancers, 2 Kaposi sarcomas, 1 lymphosarcoma, 1 breast cancer, 1 prostate cancer, 1 renal cancer, 1 rectal cancer and 2 polyps of the colon. The case fatality rate was 3.6 per cent. The importance of precision of the immunosuppressive treatment for reducing the incidence of these complications is pointed out.