ABSTRACT. An 11-month-old male mixed breed dog was euthanized due to two months history of vomiting and anorexia. At necropsy, numerous, multifocal or coalescing, firm, protruding nodules, 5 to 40 mm in diameter were scattered throughout the mesentery and omentum. Histologically and immunohistochemically, the nodules were diagnosed as malignant mesothelioma. Metastasis to the regional mesenteric, mediastinal and tracheobronchial lymph nodes were observed.KEY WORDS: canine, mesothelioma, neoplasia.J. Vet. Med. Sci. 64(3): 269-271, 2002 Mesotheliomas are neoplasms of the serous lining mesothelial cells of the abdominal, thoracic, and pleural cavities [4,6]. Spontaneous malignant mesotheliomas are rare, but sporadic cases have been documented in various animal species [4,6]. They occur most frequently as a congenital tumor in calves, but the age of onset is 4 to 13 years (mean 7.8 years) in canine [4]. Experimentally induced mesotheliomas have been reported in non-human primates and rodents following intratracheal exposure to asbestos fibers or cigarette smoke [2,3]. Mesotheliomas in animals are divided into epithelial and sclerosing types based on the histologic patterns of the neoplasm [4,6]. In the present report, we document a case of canine juvenile mesothelioma.An 11-month-old female mixed breed dog was admitted to the Seoul National University Veterinary Teaching Hospital after two months history of intermittent vomiting, anorexia, and weight loss. Pleural effusion, ascites, and abdominal masses were revealed upon explorative laparotomy. Because of the poor prognosis, the dog was euthanized, and postmortem examination was performed.At necropsy, the abdominal cavity contained a large amount of serosangineous fluid. Numerous, multifocal or often confluent, tan to white, firm, protruding nodules, 5 to 40 mm in diameter, were scattered throughout the mesentery, omentum, and serosal surfaces of the abdominal organs giving a characteristic velvety appearance (Fig. 1). Mesenteric lymph nodes were enlarged about two to four times their normal sizes. In pleural cavity, mediastinal and tracheobronchial lymph nodes were enlarged but no neoplastic nodule as seen in the abdominal cavity was found on pleural surface.Tissue samples from the abdominal tumor nodules and representative tissue specimens were fixed in 10% phosphate-buffered formalin, routinely processed, and stained with hematoxylin and eosin (H&E) for light microscopic examination. Replicate sections of the tumor masses were