Fibrohistiocytic tumors with similar histologic features may have vastly different biologic behaviors and sometimes the distinction between benign and malignant lesions may be difficult. Flow cytometry of celluar DNA content is a rapid, objective, quantitative and sensitive method to determine amlignancy in some tumors. In order to evaluate whether DNA contents and S and G2M fractions in fibrohistiocytic tumors are useful for a quantitative marker of malignancy, flow cytometric DNA analysis was done on paraffin-embedded biopsy specimens form 23 patients. These included 7 dermatofibromas, 4 dermatofibrosarcoma protuberans and 12 malignant fibrous histiocytomas (MFH). The results were as follows. l. All benign and borderline fibrohistiocytic tumors, and 6 patients with MFHs were diploidy and five of 11 patients with MFHs aneuploidy. Only one patient with aneuploid MFH died 7 months after the diagnosis. 2. S and G2M fractions were 15.2±4.696 in dermatofibromas, 17.4±10.2% in dermatofibrosarcoma protuberans, and 27.1±13.8% in MFHs. The percentage of S and G2M cells in MFHs was higher than in benign (p<0.05) and borderline lesions. Among the group of MFHs, the aneuploid cell lines showed significantly higher proliferative activity (39.1±13.5%) than the diploid ones (19.6 ±7.2%)(p< 0.001). 3. DNA aneuploidy was noted more frequently in patients with high histologic grade and large tumor size. The proportion of S and G2M cells had positive correlation with mitotic activity (r=0.4417, p<0.05) and histologic grade (r=0.4230, p<0.05). These results indicated that cellular DNA contents and percentages of S and G2M cells can serve as a quantitative marker of malignancy in fibrohistiocytic tumors and may be a useful prognostic factors in MFHs.