In each of seven ferrets (Mustela putorius furo) with leiomyosarcoma, a sin
gle dermal mass was identified and biopsied. Each mass consisted of a well-
demarcated but nonencapsulated proliferation of large spindle- to strap-sha
ped cells arranged in interwoven bundles. The cells resembled the smooth mu
scle cells of the adjacent arrector pili muscles, but with marked nuclear p
leomorphism. Immunohistochemical staining for smooth muscle actin, desmin,
and vimentin was positive and staining for myoglobin and cytokeratin was ne
gative. Follow-up on three of the ferrets indicates that the prognosis is g
ood following complete surgical excision.