In epidemiologic studies, non-Hodgkin's lymphoma (NHL) has been associated with exposure to chemicals such as phenoxyacetic acids; chlorophenols; dioxins; organic solvents including benzene, polychlorinated biphenyls, chlordanes; and immunosuppressive drugs. Experimental evidence and clinical observations indicate that these chemicals may impair the immune system. The risk is increased for NHL in persons with acquired and congenital immune deficiency as well as autoimmune disorders. Also, certain viruses have been suggested to be of etiologic significance for NHL. In some cases of NHL the common mechanism for all these agents and conditions may be immunosuppression, possibly in combination with viruses.