We report on the clinico-biological characteristics of 20 cases of gammadelta T cell large granular lymphocyte (LGL) leukemia. All the data were compared to that of 196 cases with alphabeta T cell subtype, which represents the majority of T cell LGL leukemias. Clinical findings were quite similar in the two groups regarding age, sex ratio, recurrent infections, and association with auto-immune diseases especially rheumatoid arthritis. Gammadelta LGL predominantly expressed a CD3+/CD4-/CD8+/CD16+/CD57+ phenotype, in 50% of cases. Clinical outcome was favorable for these patients with overall survival of 85% at 3 years. Fifty percent of gammadelta patients required treatment and the response to therapy was estimated at 55%. gammadelta and alphabeta T cell LGL leukemia harbor a very similar clinico-biological behavior and represent part of an antigen-driven T cell lymphoproliferation.