Malignant histiocytosis with chronic course

Summary Four cases of malignant histiocytosis with leukemic manifestations and chronic course were reported. Light microscopic, ultrastructural and ultrastructural cytochemical details of these atypical cells were demonstrated. Ultrastructurally these cells resembled hairy cells most closely among the known varieties of leukemic cells. However, ribosome-lamella complexes were not found and some atypical cells had a few short cytoplasmic projections. In addition, tartrate-resistant acid phosphatase was absent from these cells. We speculate that this leukemic reticuloendotheliosis with a chronic course seen in Japan seems to be analogous to malignant histiocytosis with massive splenomegaly reported by Vardiman et al.