OBJECTIVE: To assess risks for developing second malignancies in patients with mycosis fungoides or Sézary syndrome. DESIGN: Retrospective study of 2 cohorts. SETTING: Nine population-based US cancer registries that constitute the Surveillance, Epidemiology, and End Results Program (SEER-9), and Stanford University referral center cohort of patients with cutaneous lymphoma. Patients with mycosis fungoides or Sézary syndrome from the SEER-9 registry diagnosed and followed up from 1984 through 2001 and from the Stanford University cohort diagnosed and followed up from 1973 through 2001. MAIN OUTCOME MEASURES: Relative risk was estimated using the standardized incidence ratio (SIR). The expected cancer incidence for both cohorts was calculated using age-, sex-, race-, and calendar year-specific SEER-9 incidence rates for the general population. Nonmelanoma skin cancers were excluded because these cancers are not routinely reported by the SEER database. RESULTS: In the SEER-9 cohort (n = 1798), there were 197 second instances of cancer (SIR = 1.32; 95% confidence interval [CI], 1.15-1.52) at all sites. Significantly elevated risk (P<.01) was observed for Hodgkin disease (6 cases; SIR = 17.14; 95% CI, 6.25-37.26) and non-Hodgkin lymphoma (27 cases; SIR = 5.08; 95% CI, 3.34-7.38). Elevated risk (P<.05) was also observed for melanoma (10 cases; SIR = 2.60; 95% CI, 1.25-4.79), and urinary cancer (21 cases; SIR = 1.74; 95% CI, 1.08-2.66). In the Stanford University cohort (n = 429), there were 37 second instances of cancer (SIR = 1.04; 95% CI, 0.76-1.44). Elevated risk (P<.01) was observed for Hodgkin disease (3 cases; SIR = 27.27; 95% CI, 5.35-77.54). Elevated risk (P<.05) was also observed for biliary cancer (2 cases; SIR = 11.76; 95% CI, 1.51-42.02). CONCLUSION: Updated SEER (population based) and Stanford (clinic based) data confirm the generalizability of earlier findings of increased risk of lymphoma in patients with mycosis fungoides or Sézary syndrome.