The concept of soft tissue sarcomas composed predominantly of myofibroblasts has been controversial. We examined a series of such lesions of low- and intermediate-grade malignancy to further define their clinical and pathologic features. Histologic appearances of four cases diagnosed as myofibrosarcoma by electron microscopy were reviewed. Eleven additional cases with similar morphology were then identified from 249 tumors originally indexed as fibrosarcoma. Electron microscopy was performed on five of these, and immunohistochemistry was carried out on all cases for which material was available. There were 11 men and 4 women aged 33 to 73 years (median, 54 yrs; mean, 53 yrs). Lesions mainly involved the head and neck, extremities, and trunk and ranged in size from 1.5 to 12 cm. The tumors were composed of bland or pleomorphic stellate to spindled cells with eosinophilic cytoplasm and tapered nuclei in a collagenous stroma; 10 were grade 1 and five were grade 2. All sarcomas displayed fascicular or storiform patterns, and some of the grade 1 lesions superficially mimicked nodular fasciitis. Electron microscopy of nine cases showed myofibroblastic differentiation, and immunohistochemistry showed smooth muscle actin in 13 of 15 cases, muscle-specific actin in 7 of 9, desmin in 6 of 14, and cytokeratin in 0 of 11. Four of nine grade 1 and three of four grade 2 tumors recurred (one twice), and one grade 2 tumor metastasized to the lungs. Myofibrosarcomas are indolent low-grade or occasionally aggressive intermediate-grade sarcomas which can be recognized by light microscopy. Their clinical importance lies in the resemblance, particularly of low-grade examples, to reactive or pseudosarcomatous conditions.