Among 92 dairy goats, mainly of Saanen breeding, necropsied during studies on slow viral diseases, 17 (25.3%) of 67 over 2 years old had thymoma. Ten were females and seven were castrated males. In none was the thymoma accompanied by clinical signs attributable to it. Except for one at the thoracic inlet, all tumors were in the cranial mediastinal cavity. Encapsulated and sometimes cystic, they varied from a 2-centimeter nodule to a smoothly bosselated 10x8x8-centimeter mass weighing 300 grams. Although they were composed of lymphocytes and epithelial cells, small lymphocytes predominated in 16. One was predominantly epithelial. Hassall's corpuscles and myoid cells were seen in all tumors. In 12 tumors, small veins were encircled by spaces that varied from empty clefts to wide, fluid-filled cavities. The high prevalence of thymoma in these goats supports previous reports that this tumor occurs more often in Capra hircus than in other domestic animals. It may be one of the more common neoplasms in older goats.
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