Myxofibrosarcoma/myxoid malignant fibrous histiocytoma (MFH) has continued to be considered a distinct entity even after recently published reassessments of pleomorphic sarcomas and MFH. Several cell cycle-regulated proteins have already been screened by immunohistochemistry with the aim of finding the reliable prognostic indicator of soft tissue sarcomas; however, it is still unknown whether their altered expression affects patient survival in myxofibrosarcoma. In this study, we evaluated the expression of p53, MDM2, MIB-1 (Ki-67), p21, p27, p16, cyctin A, cyclin D1, and cyclin E by immunohistochemistry in 45 cases of myxofibrosarcoma. First, we searched for possible clinicopathologic prognostic factors in 61 cases of myxofibrosarcoma for which follow-up data were available. In univariate analysis, large tumor size (greater than or equal to5 cm), deeply situated tumor, and high histological grade (grade 2 or 3) significantly decreased survival (log-rank test, P < 0.05). Among 43 cases of myxofibrosarcoma for which immunohistochemical findings were available, high MIB-1 labeling index (LI) (cutoffs of 10 and 22.5 on average), high cyclin A LI (cutoffs 10% and 13.8% on average), low p21 LI (cutoffs 10 and 20.7 on average), and reduced abnormal expression of p16 were adverse prognostic factors. In multivariate analysis (Cox proportional hazards model), high mitotic rate (>15/10 high-power fields), p53 immunoreactivity (cutoff 10%), high MIB-1 LI (>22.5), low p21 LI (<20.7), and low p27 LI (<47.8 on average) were independent poor prognostic factors. Our results suggest that reduced expression of p21 could be considered a new parameter to be evaluated, along with classical clinicopathologic prognostic factors, for identifying those at high risk for myxofibrosarcoma. (C) 2003 Elsevier Inc. All rights reserved.