The clinical, morphological, immunologic, and cytogenetic features of seven cases of chronic granulated T cell lymphocytosis with neutropenia were studied. The disorder was characterized by moderate blood and bone marrow lymphocytosis, neutropenia, polyclonal hypergammaglobulinemia, splenomegaly, absence of lymphadenopathy, and a chronic, relatively stable clinical course. The proliferative lymphocytes manifested a cytotoxic/suppressor T lymphocyte phenotype. In two of four cases studied, blood lymphocytes showed clonal chromosome abnormalities. One patient treated with pulse steroid therapy had reversal of lymphocytosis and severe neutropenia with subsequent resolution of an intractable infection. The lymphocytosis and neutropenia recurred when steroids were withdrawn. Six of the seven patients were living three months to 17 years from diagnosis; one died at 4.3 years of an unrelated cause. Five of the patients, including the two with lymphocyte chromosome abnormalities, had persistent lymphocytosis and neutropenia from three months to 13 years from diagnosis. In two patients, the disease appears to have undergone spontaneous regression. No differences in clinical presentation or the morphological or immunologic characteristics of the proliferative lymphocytes were apparent between those patients with lymphocyte chromosome abnormalities and persistent disease and those who had a spontaneous regression. The finding of clonal chromosome abnormalities in the blood lymphocytes of two of the patients in this study suggests a neoplastic origin for chronic granulated T cell lymphocytosis with neutropenia. However, apparent spontaneous regression in two patients, one after 11 years, lends support to a chronic reactive or immunoregulatory disorder as the etiology. It is probable that cases of granulated T cell lymphocytosis with neutropenia, although morphologically and immunologically similar, are biologically heterogeneous.