S100-positive T lymphocytes account for less than 3% of peripheral blood T cells. Rare cases of S100-positive T-cell lymphoma have been previously described. We report four such cases of S100-positive T-cell chronic lymphoproliferative disease. In all cases, hepatosplenomegaly was observed, without prominent lymphadenopathy. Central nervous system (CNS) involvement by the leukemic cells was suggested in three cases by physical symptoms and confirmed in two cases by cerebrospinal fluid studies. Despite treatment, three patients died at 3, 6, and 8 months after diagnosis. Although there was a leukemic presentation, only minimal bone marrow infiltration was evident. Splenectomy showed red pulp infiltration. Liver and lymph node biopsies showed sinusoidal leukemic involvement. In all cases, the leukemic cells expressed mature T-cell- and natural killer cell-associated antigens. Cytoplasmic S100 was detected in the leukemic cells in the blood, spleen, liver, and lymph node. Southern blot studies in two cases showed T-beta, T-gamma, and T-delta gene rearrangements. RNA Northern blots showed T-alpha and T-beta chain transcripts with no T-gamma or T-delta RNA identified. Southern blot analysis showed no hybridization to probes specific for Epstein-Barr virus, cytomegalovirus, human immunodeficiency virus-1, or human T-cell lymphotropic virus type-1. These findings show that S100-positive T-cell chronic lymphoproliferative disorder is an aggressive, extramedullary-based disease frequently associated with CNS involvement and characterized by short survivals.