Four of 105 patients with chronic lymphocytic leukemia (CLL) manifested clinical, morphologic, ultrastructural and membrane surface marker characteristics that differed from those found in patients with typical CLL of demonstrated B-lymphocyte origin. These four patients presented with moderate increases In absolute lymphocyte counts, absolute neutropenia, polyclonal hypergammaglobulinemia and hepatosplenomegaly without lymphadenopathy. Two of them were unusually young, 19 and 25 years old, at the time of diagnosis. The proliferating lymphocytes carried receptors for sheep erythrocytes, a T-lymphocyte marker. In the three patients tested, the lymphocytes also carried Fc receptors. Ultrastructurally the lymphocytes contained cytoplasmic inclusion bodies consisting of parallel tubular arrays. The parallel tubular arrays corresponded to prominent cytoplasmic azurophilic granules on light microscopy. Parallel tubular arrays were found in less than 1 per cent of the lymphocytes in eight patients with typical B-lymphocyte CLL. The process in these four patients may be a distinctive chronic lymphoproliferative disorder originating in T lymphocytes with Fc receptors found in small numbers in the blood of normal persons.