A five-year-old Standardbred trotter had lost a signifi cant amount of weight over a period of a few weeks. The left forelimb was swollen, and the mare was lame at a walk. Pain was localized in the shoulder area, and the muscles of the lateral aspect of the shoulder appeared hard in palpation. Ultrasonographic findings of the left triceps muscle consisted of muscle fibers replaced by anechoic loculated fluid and echogenic masses. Findings strongly suggested a neoplastic disorder. Biopsies from the affected area were taken twice at a two-week interval, but showed only unspecific inflammatory changes. The final diagnosis of hemangiosarcoma was obtained in necropsy. No metastases were identified.