BACKGROUND: Clonal diseases of large granular lymphocyte (LGL) disorders can arise from a CD3+ T-cell lineage or from a CD3- NK-cell lineage. CD3+ LGL leukemia is the most frequent form of LGL leukemia and is a distinct entity by FAB and REAL classifications. METHODS: The clinical course, biological features, and recent data on pathogenesis of CD3+ LGL leukemia are reviewed. The spectrum of differential diagnosis is described. RESULTS: T-LGL leukemia affects elderly people. Approximately 60% of patients are symptomatic; recurrent infections secondary to chronic neutropenia, anemia, and rheumatoid arthritis are the main clinical features. The most common phenotype is CD3+, CD8+, CD57+. Clonality is detected by clonal rearrangement of the T-cell receptor gene. Clinical and molecular remission can be obtained with oral low-dose methotrexate. Serologic findings show frequent reactivity to the BA21 epitope of HTLV-I env p21e, suggesting that a cellular or retroviral protein with homology to BA21 may be important in pathogenesis. Clonal expansion may be facilitated by IL-12 and IL-15 lymphokines. Constitutive expression of Fas ligand by leukemic LGLs support the hypothesis that leukemic cells arise from antigen-activated cytotoxic T cells. Leukemic LGLs express a multidrug-resistance phenotype that could partly explain the chemoresistance observed in aggressive cases. CONCLUSIONS: CD3+ LGL leukemia is a distinct lymphoproliferative T-cell disorder with specific clinicobiological aspects. The clinical spectrum of LGL proliferations is wide and immunophenotypic, and genotypic studies are needed to establish the diagnosis.