Two cases of smooth muscle tumor of the breast with high tendency to l
ocal relapses are reported. The common cytohistologic feature of these
infrequent neoplasms is interfacing bundles with spindle-shaped cells
showing eosinophilic cytoplasm and blunt-ended nuclei. In estimating
the prognosis of these lesions, the mitotic index must be evaluated to
gether with evidence of necrosis and hypercellularity. The present cas
es suggest that, in the absence of the ancillary features, a mitotic r
ate of 1-3 x 10 HPF might define a ''grey zone'' in the spectrum of le
sions between leiomyomas and leiomyosarcomas: the smooth muscle tumors
of indeterminate prognosis (low risk lesions).