Background Myofibroblastic sarcoma was used to be a controversial neoplasm.This study investigated theclinicopathological features of 20 cases of myofibroblastic sarcoma arising in different locations.Methods The paraffin-embedded tissue samples from 20 cases of patients with myofibroblastic sarcoma were stainedimmunohistochemically,and 5 cases examined by electron microscopy.Student’s t test was used to analyze thedifference of Ki-67 labeling index between grade 1 and grade 2 myofibroblastic sarcomas.Results Histologically,the tumors were composed of slender spindle cells with eosinophilic cytoplasm,and fusiform,tapering,wavy,or plump ovoid;vesicular nuclei and a small central eosinophilic nucleoli.Immunohistochemically,thetumor cells expressed smooth muscle actin (18/20),muscle specific actin (16/20),fibronectin (20/20) and desmin (2/20).Ultrastructurally,the tumor cells revealed abundant rough endoplasmic reticulum and longitudinally arranged finefilaments with focal densities in the cytoplasm.A clinical follow-up of 19 patients showed that 2 cases experienced localrecurrence and distant metastasis 6 months to 4 years after the initial operation.Nine cases recurred locally 17 to 46months after the initial excision,and 9 cases were alive with no evidence of disease.Conclusions Myofibroblastic sarcomas,which exhibit diverse histological appearance,can easily be misdiagnosed asbenign tumors.Myofibroblastic sarcomas are local destructive lesions with frequent recurrence,and may metastasedistantly.Chin Med J 2007;120(5):363-369 Background Myofibroblastic sarcoma was used to be a controversial neoplasm. This study investigated the clinicopathological features of 20 cases of myofibroblastic sarcoma arising in different locations. Methods The paraffin-embedded tissue samples from 20 cases of patients with myofibroblastic sarcoma were stainedimmunohistochemically, and 5 cases examined by electron microscopy. Stude’s t test was used to analyze the difference of Ki-67 labeling index between grade 1 and grade 2 myofibroblastic sarcomas. Results Histologically, the tumors were composed of slender spindle cells with eosinophilic cytoplasm, and fusiform, tapering, wavy, or plump ovoid; vesicular nuclei and a small central eosinophilic nucleoli. Immunohistochemically, the tumor cells express smooth muscle actin (18/20), muscle specific actin (16/20), fibronectin (20/20) and desmin the tumor cells have abundant rough endoplasmic reticulum and longitudinally arranged finefilaments with focal densities in th e cytoplasm. A clinical follow-up of 19 patients showed that two cases experienced local recurrence and distant metastasis 6 months to 4 years after the initial operation. Nine cases recurred locally 17 to 46 months after the initial excision, and 9 cases were alive with no evidence of disease.Conclusions Myofibroblastic sarcomas, which exhibit diverse histological appearances, can easily be misdiagnosed as benign tumors. Myofibroblastic sarcomas are local destructive lesions with frequent recurrence, and may metastasedly. Chin Med J 2007; 120 (5): 363-369