Chromosome and pathologic studies were performed on two patients (a 12-year-old boy and a 62-year-old woman) with malignant histiocytosis (MH). Both patients had chromosome abnormalities in their neoplastic cells: the boy's karyotype was 45,Xp+,-Y,9p+,18q-, and the woman's 48,XX,+16, inv(1),mar(5),6p-,10p+,12q+,i(18q),+i(18q). The boy had typical clinical and pathologic findings of MH, and died without achieving remission by chemotherapy. At the initial stage the woman had the clinical and hematologic findings of MH. Chemotherapy was given, but had no beneficial effects. At the terminal stage the bone marrow (BM) biopsy and aspirate, and the autopsy findings, were consistent with those of non-Hodgkin lymphoma, diffuse large cell type, although some histiocytes or abnormal cells in monocyte-macrophage lineage remained in the BM and the autopsied lymph nodes. This study and a review of data on six other cases have failed to establish any specific chromosome changes in MH.