Malignant histiocytosis (MH) was diagnosed on the cytologic and cytochemical features of the malignant cells present in bone marrow smears from an infant and a child. The diagnosis of MH was confirmed by light and electron microscopic studies on bone marrow and skin biopsy specimens, and bone marrow and liver biopsy specimens, respectively. Both patients showed a deterioration while receiving prednisone monotherapy, but they responded well to a combination of vincristine and cyclophosphamide. The infant has remained disease-free for 52+ months now, but the child died of a relapse 11 months after diagnosis. Cytogenetic studies of blood and/or bone marrow cells were performed before treatment. In the infant, a pathologic cell line with a translocation t(8;16)(p11;p13) was found; this abnormality was no longer present after remission was obtained. In the second patient, a hyperdiploid cell line with major karyotypic anomalies was found. When studied in relapse and shortly before death, additional chromosomal abnormalities were seen. The data from this study show that prednisone should be used with caution in MH, and that it should be omitted from combination chemotherapy when adverse effects are noted during short-term monotherapy. Also, cytogenetic studies should be performed more often in MH to determine the significance and possible nonrandomness of chromosomal abnormalities in this disease.