Large series of patients with Sézary syndrome (SS), the leukemic variant of cutaneous T-cell lymphoma (CTCL), have been reported infrequently because of its low incidence. Here we recorded several clinical, histopathological and immunophenotypical features of 29 cases of leukemic CTCL patients from four Dermatology Departments of Catalonia, Spain, and analyzed their prognostic value. Clinical data included sex, age, delay of SS diagnosis, previous diagnosis of lymphoma, B-symptoms, type of skin lesions, peripheral adenopathy, histologic evaluation of lymph node biopsy, visceral involvement, percentage of circulating Sézary cells, serum LDH and beta-2-microglobulin levels, first treatment and response, disease-free interval, further therapies and survival. Histopathological data examined were epidermotropism, depth and thickness of the infiltrate, cell size, adnexal involvement, presence of granuloma, eosinophils and plasma cells, mitotic rate. The percentage of CD45Ro, CD43, CD20, CD30 and CD8 positive dermal cells were also recorded. Survival showed a mean actuarial risk of 57% at 3 years and 38% at 5 years, with a median survival of 48 months. Analysis of actuarial survival demonstrated as following as features linked with a bad prognosis: fast evolution of the disease (from symptoms onset up to diagnosis) (p = 0.0274) raised levels of serum lactate dehydrogenase (p = 0.0379) and beta-2-microglobulin (p = 0.0151), the latter being the most important prognostic factor. In conclusion although SS had been traditionally considered as a low-grade lymphoma, the present study agrees with the recent classification rating SS as an aggressive type of CTCL with a poor prognosis. Our results show that some simple clinical and blood test data can be useful as prognostic indicators in this disease.