The purpose of this prospectively collected single center study cohort of 1,263 patients with mycosis fungoides (MF)/Sézary syndrome (SS) is to evaluate the significance of stage and risk of disease progression from initial presentation and to examine other prognostic factors. The prognostic variables effecting overall survival (OS) were examined in a unique prospective cohort of 1,263 patients with MF and SS seen by one investigator at MD Anderson Cancer Center (Houston, TX) from 1982 to 2009. Kaplan-Meier estimates were used to determine median OS, progression-free survival (PFS), and disease-specific survival (DSS). Cox proportional hazards regression model assessed prognostic factors. Mean age at diagnosis was 55.33 years. Early mycosis fungoides (stage IA-IIA) represented 71.5% (903 of 1,263) and advanced (stage IIB-IVB) 28.5% (360 of 1,263) patients. Progression to a higher stage occurred in 147 patients (11.6%) of whom 112 (12%) were early and 35 (9.7%) advanced. Death from disease occurred in 102 of 1,263 (8.1%) patients. Median OS was 24.44 years, PFS was 16 years, and median DSS was not reached. OS and PFS were significantly better for early-stage patients with patches (T1a/T2a) than with patches/plaques (T1b/T2b). The PFS analyzed in 1,241 patients found that only 337 (27.2%) had disease progression or had died from disease. Risk factors associated with progression or deaths were advanced age, plaque stage, lactate dehydrogenase (LDH) level, and tumor area. Improved outcome of MF/SS, reflected by OS and PFS for all stages, may result from earlier diagnosis, new therapies, and aggressive treatment of infections. ©2012 AACR.