Background: The influence of prognostic factors on survival in patients with mycosis fungoides/Sézary syndrome (MF/SS) is less well described than for other lymphomas. Objective: Our purpose was to evaluate the prognostic value of diverse clinicopathologic and laboratory characteristics in patients with MF/SS. Methods: All 115 patients with MF/SS seen at the Mycosis Fungoides clinic at M. D. Anderson Cancer Center during the study period who had slides available for pathologic review were analyzed. Univariate and multivariate methodologies were used. Results: Age (≥ 60 years; P = .0002), advanced stage (P < 10^-5 ), tumor (T3) stage disease (P ≤ 10^-5), lymphadenopathy (P = .006), bone marrow infiltration (P = .03), high lactate dehydrogenase (LDH; P = .0002), high β₂-microglobulin (> 2 mg/L; P = .009), and transformation to large-cell lymphoma (P = .004) were significant prognostic factors in the univariate analysis and correlated with a poorer survival. The outcome of patients staged as IIB was significantly worse than that of those staged as I or IIA or III (P < .001) and was comparable to that of the patients staged as IV (P = .8). In the multivariate analysis, the factors selected include stage (I to IIA and III vs IIB and IV; P < .0001), LDH ( P = .006), and age (≥ 60 vs < 60 years; P = .02). The actuarial median survival of patients with advanced stage, high LDH, or age 60 years or more was 2.5 to 3.5 years, whereas that of patients without any of these parameters was more than 13 years. Conclusion: Our results suggest that patients with MF/SS who are staged as IIB or IV, who have a high LDH, or who are 60 years of age or older have an aggressive course and poor survival. (J Am Acad Dermatol 1999;40:914-24.)