Clonal expansions of CD3+ large granular lymphocytes (LGL) have been classified as T-LGL leukemia. The majority of patients with T-LGL leukemia have a chronic disease (years) manifested often by severe neutropenia, rheumatoid arthritis, and mild-to-moderate splenomegaly. The characteristic phenotype of the leukemic LGL is CD3+, CD8+, CD16+, CD57+, and CD56-. In this report we describe an aggressive variant of T- LGL leukemia in which leukemic LGL also expressed CD56, as identified by two-color flow-cytometry analysis. In contrast to the chronic nature typical of T-LGL leukemia, these patients presented with a severe systemic illness that was rapidly progressive and resistant to treatment. Atypical clinical features included rapidly increasing spleen size to massive proportions, extensive lymphadenopathy, and the presence of B symptoms (fever, nightsweats, weight loss). Hematologic and pathologic features were also unusual for T-LGL leukemia. These patients had very high LGL counts at diagnosis (range 11,692 to 26,312 microL), which increased rapidly despite treatment. Histopathologic examination of splenic sections showed extensive infiltration of red pulp cords and sinuses by leukemic cells with atrophy of the white pulp. These clinicopathologic features are similar to those described for patients with natural killer cell (NK)-LGL leukemia, whose cells are also CD56+. However, unlike NK-LGL leukemia, we could not show a direct pathogenic role for Epstein-Barr virus (EBV), as Southern-blot analyses using an EBV-joined termini probe were negative in these patients. Our findings suggest that CD3+, CD56+ LGL leukemia is a distinct clinicopathologic entity separate from the usual CD3+, CD56- T- LGL leukemia. The expression on leukemic LGL of CD56, an adhesion molecule, may determine the aggressive biologic nature of this newly described disease.