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A 25-year-old stallion was examined for unilateral scrotal swelling and weight loss. Physical and ultrasound examinations revealed a painless, enlarged left testicle with non-homogeneous echogenicity when compared with the contralateral testicle. Grossly, the left testicle was irregularly enlarged (20x13x30 cm) and firm. Transrectal palpation revealed nodular structures in the entire pelvic cavity and in the peritoneum of the caudal abdominal cavity. Abdominocentesis showed an increased amount of red abdominal fluid with altered laboratory parameters (leukocytes 5,8 G/l, total protein 36 g/l). Findings from clinical examination, transrectal palpation, abdominocentesis and thoracic radiographs were suggestive of a testicular neoplasia with metastases and the horse was euthanized. Necropsy revealed that the testicular parenchyma was replaced by a white-yellowish tumor mass. Histopathologically, the diagnosis of a malignant mixed sex cord-stromal tumor was made. Metastases of the tumor were found in numerous organs including the spleen, liver, lung, kidney, gastrointestinal tract and the contralateral testis as well as in peritoneum, pericardium, pleura and the bone marrow. A mixed sex cord-stromal tumor in the stallion is extremely rare. Distant metastases of this tumor have not been described previously.