Diverse states of immunodeficiency are complicated by the development of various types of cancer. Most striking in nearly every type of immunodeficiency is a high incidence of a neoplasm of the immune system itself, the NHL, which consists of activated pathologic lymphocytes. Morphologically, these are mostly reticulum cell sarcomas, arise from B-cells, tend to be extranodal in distribution, have a predilection for the brain and are probably caused by EBV. If Kaposi's sarcoma is a form of lymphoreticular malignant disease, as some investigators think, the situation becomes even more intriguing. We need to intensively study the various groups of immunosuppressed patients, including those with and without tumors, to seek clues to the cause of neoplasms. Such information may shed light on the causes of similar malignant conditions that occur in the general population and on the role of the immune system in the control of cancer. Hopefully, such knowledge may lead to immunologic methods for the prevention and cure of neoplasms.