The history of occupational, environmental, and/or iatrogenic exposure to potential carcinogenic agents was obtained at the time of onset of skin disease in 43 of 44 patients with cutaneous T-cell lymphoma (mycosis fungoides and the Sézary syndrome) entering a National Cancer Institute therapeutic trial. A history of multiple exposures to these agents was common, the two most frequent being chemicals (91% of patients) and drugs (86%). Mean duration of expsosure was 13 years for chemicals and 18 years for drugs. The most common chemicals were air pollutants (39%), pesticides (36%), solvents and vapors (30%), and detergents and disinfectants (14%). Increased severity of disease was seen with increaed duration of chemical exposure in stage V cutaneous T-cell lymphomas only. The most frequent drugs besides tobacco (86%) were analgesics (20%), tranquilizers (18%), and thiazides (14%). Second cancers occurred in four patients, including two renal cell carcinomas, and a family history of cancer was present in 11. Fourteen of 22 patients questioned had recurrent herpes simplex. Patients with chronic skin disease who have long-term exposure to combinations of chemicals, physical agents, and biologic agents, are heavy smokers, or have recurrent herpes simplex appear to be prime candidates for developing mycosis fungoides or the Sézary syndrome.