We present the cytologic, histologic, immunohistochemical, and ultrastructural features of two cases of myofibroblastoma with different cellularity, one paucicellular the other hypercellular. Fine-needle aspiration suggested the diagnosis in the case of the hypercellular tumor of the breast and pointed out the benign nature of the paucicellular case, arising in the breast of a young woman. Immunohistochemistry, showed smooth muscle-specific actin, vimentin, and desmin positivity consistent with myofibroblastic differentiation, in both cases. The positivity of both tumors to CD34 suggests that they may be related to the so-called solitary fibrous tumor. In addition, there was strong progesterone- and weak estrogen-receptor positivity ion the hypercellular case, a novel finding in these tumors, with possible histogenetic implications. Ultrastructural findings in the first case were consistent with the myofibroblastic origin of the tumor.
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