The clinicopathologic features of 23 patients with hematophagic histiocytosis (HH) are described. All of them exhibited increased histiocytes associated with hemophagocytosis in the marrow. The patients usually presented with fever, hepatosplenomegaly, lymphadenopathy, and cytopenia. The underlying illnesses were heterogeneous, including non-Hodgkin's lymphoma in 17, systemic lupus erythematosus in one, diabetes mellitus in one, acute myelomonocytic leukemia in one, myelodysplastic syndrome in one, and unknown cause in two. Among 17 non-Hodgkin's lymphoma, 14 were peripheral T-cell lymphoma, two were B-cell lymphoma, and one was an undefined phenotype. Among 14 patients with peripheral T-cell lymphoma, six of the patients had nasal T-cell lymphoma. Five of these 14 patients initially diagnosed as malignant histiocytosis turned out to be T-lineage lymphoma after immunophenotypic studies. Active infections, most of viral origin, were documented in eight patients, including Epstein-Barr virus in three, cytomegalovirus in three, herpes simplex virus in three, Pseudomonas aeruginosa in one, Bacteroides vulgatus in one, and mycoplasma in one. Some of them had mixed virus and bacteria infection. Sixteen (70%) of our patients died of their acute illness within 10 weeks of the diagnosis of HH. In the past, the clinical and histologic differentiation between hematophagic histiocytosis and true histiocytic neoplasm (histiocytic medullary reticulosis/malignant histiocytosis) has proved difficult, but now these can be distinguished with immunohistologic, immunogenetic, and cytogenetic studies, especially in the cases of peripheral T-cell lymphoma with hemophagocytic syndrome.