The incidence of malignant disease in AIDS patients is around 40%. Ninety percent of the tumors are Kaposi's sarcoma and 10% are malignant lymphomas, predominantly of B cell origin. The natural history of the AIDS-associated Kaposi's sarcoma is variable. It may remain entirely asymptomatic without treatment for many months or it may be rapidly progressive and produce life-threatening pulmonary, cardiac, or gastrointestinal symptoms requiring acute therapeutic intervention. The malignant lymphomas occurring in AIDS patients are usually very aggressive in their natural history. The treatment of choice for symptomatic Kaposi's sarcoma and malignant lymphoma is chemotherapy. In general, such treatment can alleviate symptoms and improve the quality of life in AIDS patients; however, the underlying immune defect is not improved by such treatment and the patients remain susceptible to life-threatening opportunistic infections. The ultimate control of the malignancies associated with AIDS depends upon the development of therapies capable of reversing the underlying immune defect.