A distinctive, benign, spindle cell tumor of the breast is described. Three patients were men, the fourth a woman. Histologically, each tumor displayed islands of spindle or stellate cells embedded in a collagenous matrix. Within the female breast, the lesion was within the stroma, displacing but not including the epithelial elements. Ultrastructural study of two tumors revealed diverse populations composed of fibroblasts, myofibroblasts, smooth muscle cells, and undifferentiated mesenchymal cells. The relationship of these tumors to spindle cell lipomas is discussed. The lesions have all been clinically benign, and their recognition as such is important
