The course of mycosis fungoides (MF) is indolent except when transformation to a large T-cell lymphoma occurs. The diagnosis of transformed MF (T-MF) relies on the presence of more than 25% of large cells on biopsy of an MF lesion. We analyzed 45 patients with T-MF recorded by the French Study Group on Cutaneous Lymphomas to better determine clinicopathological features of MF transformation and to analyze their impact on prognosis. Median time from diagnosis of MF to transformation was 6.5 years. Extracutaneous progression was present in 20 patients. Mean survival from transformation to death was 22 months. In univariate analysis, only an extracutaneous progression was associated with a worse prognosis (5-year actuarial survival: 7.8% versus 32%). Neither sex, age, clinical and skin disease stage at transformation, transformation speed, nor percentage of large cells or CD30 expression (14 of 45) had a prognostic value. When performing multivariate analysis, age (at least 60 years), and extracutaneous spreading were found to be associated with a poor prognosis. There was no difference between survival curves of patients with T-MF and with pleomorphic large T-cell CD30- lymphomas. The main diagnostic pitfall was "histiocytic-rich" MF, requiring CD68 staining for the diagnosis of T-MF. Out of 45 patients, 6 presented an histologic transformation before clinical progression, suggesting that an early histopathological diagnosis may be performed by histological follow-up. The prognostic value of such early histopathological diagnosis must be confirmed by prospective studies.