Various disorders of immune competence are associated with increases in uncommon tumors, particularly non-Hodgkin's lymphomas (NHLs), which often exhibit unusual features: they are frequently extranodal, show a broad spectrum of lesions, ranging from benign polyclonal hyperplasia to frankly malignant monoclonal lymphomas, and are frequently localized to the brain. Of 7136 tumors in organ transplant recipients, the predominant lesions are NHLs, carcinomas of the skin and lips, carcinomas of the vulva/perineum, Kaposi's sarcoma (KS), and renal carcinomas. Skin cancers present unusual features: predominance of squamous cell carcinomas, young age of the patients, and a high incidence of multiple tumors. Cancers of the vulva/perineum occur at a much younger age than in the general population. In AIDS patients the most common malignancy is KS, which occurs in 14%, whereas NHLs afflict 2.9%. A variety of other tumors occur in these patients. The most common tumors in patients with primary immunodeficiency states are NHLs (49%), leukemias (13%), various carcinomas (9%), and Hodgkin's disease (7%).