Thymoma was diagnosed in 15 dogs at Angell Memorial Animal Hospital between 1972 and 1983. All thymomas developed in the cranial portion of the mediastinum. An autoimmune paraneoplastic syndrome was observed in 10 (67%) of the dogs and included myasthenia gravis, nonthymic neoplasms, and polymyositis. Clinical signs were variable and inconsistent, depending on whether they were attributable to the cranial mediastinal mass or to the paraneoplastic syndrome. Eleven dogs were necropsied. Two thymomas had gross characteristics of malignancy. In 3 cases, a cell consistent with a subclass of mast cell was found and in 1 thymoma, melanocytes were observed.