A 10-month-old male infant underwent splenectomy because of life-threatening hypersplenism due to malignant histiocytosis. The spleen tissue positive for herpes simplex virus (HSV)-DNA, histologically and immunologically revealed a marked B cell depletion replaced by proliferation of activated cytotoxic T cells. S100-positive histiomonocytoid cells and lysozyme-positive hemophagocytes were also significantly increased. No metaphases were obtained by cytogenetic studies and Southern blot analysis of spleen cell DNA demonstrated only germ bands of immunoglobulin and both T gamma and beta genes, providing no evidence of monoclonality in this case of so-called malignant histiocytosis.