A 12-year-old Peruvian Paso stallion was examined for scrotal swelling and unilateral testicular enlargement of 4-6 weeks duration. Physical examination and ultrasonography of the scrotum revealed an enlarged, nonpainful right testis with a well encapsulated mass at the cranial-medial aspect containing echodense flocculent material. Unilateral castration was performed and the testis, spermatic cord and vaginal tunic were submitted for histological examination, which revealed a mastocytoma with accompanying evidence of malignancy. Short-term follow-up examination did not reveal any evidence of abdominal metastasis and the stallion has since successfully achieved pregnancy in a small number of mares. Although considered rare, mast cell neoplasia should be considered as a differential diagnosis when unilateral testicular enlargement is encountered.

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