Prior reports indicate that patients with chronic lymphocytic leukemia (CLL) may be at increased risk of subsequent neoplasms. This study quantified the risk of second cancers among 16 367 patients with CLL in the population-based Surveillance, Epidemiology and End Results Program. Overall, the observed/expected ratio (O/E) was 1.20 (95% confidence interval [CI], 1.15-1.26). Second cancer risks for patients who received chemotherapy only as the first course of treatment (O/E = 1.21) were similar to risks for those who received no treatment initially (O/E = 1.19). Significant excesses were found for Kaposi sarcoma (O/E = 5.09), malignant melanoma (O/E = 3.18), and cancers of the larynx (O/E = 1.72) and the lung (O/E = 1.66). Increased risks were also found for brain cancer among men (O/E =1.91) and for cancers of the stomach (O/E = 1.76) and bladder (O/E = 1.52) among women. Additional investigations of cancers after CLL are needed to explore the role of immunologic impairment and/or other etiologic influences.