Risk of second primary malignancy was assessed in a population-based follow-up survey of all persons who developed cutaneous T-cell lymphoma (CTCL) in nine geographic areas of the United States covered by the Surveillance, Epidemiology and End Results (SEER) program of the National Cancer Institute during the period 1973 to 1983. Among 544 patients with a first primary tumor reported as CTCL, a second cancer developed in 35 (6%), yielding a significantly elevated relative risk (RR) of 1.7, which reflects excesses for cancers of the lung and colon and non-Hodgkin's lymphoma. Although the excess of lymphoma may be related to the evolution of CTCL to less differentiated T-cell lymphoma, additional studies are needed to clarify the immunologic, genetic, viral, and environmental factors that may contribute to the development of second cancers.