To evaluate the immunologic surveillance theory of cancer, we reviewed the epidemiologic observations that have been made on cancer risk among population groups with immune deficiency. Lymphoproliferative neoplasms predominate in various groups, most notably renal transplant recipients treated with immunosuppressive agents and patients with primary immunodeficiency syndromes. In some immune disorders, specific forms of nonlymphoid neoplasia seem to occur excessively, although the patterns are not clear-cut or consistent. The available epidemiologic evidence fails to support the concept that immunosurveillance mechanisms are generally involved in carcinogenesis but does provide clues to immunologic processes that may predispose to particular neoplasms.