An 8-year-old mixed-breed dog was evaluated for caudal paresis. Transient lameness of the left hind and left forelimbs had developed during the preceding week. Clinical findings included conscious proprioceptive deficits, hyporeflexive tendon reflexes and decreased pain perception, coolness in the hind limbs and left forelimb, and absence of femoral pulses. A fluid-dense mass was radiographically identified adjacent to the left atrium. Echocardiography revealed a mass in the left atrium and spontaneous contrast in the left ventricular lumen and aortic root. The dog was euthanatized because of its deteriorating condition. A large mass was adhered to the dorsal left atrial wall and had eroded into the atrial lumen. A sterile blood clot was attached to this site, and sterile thrombi were in the terminal portion of the aorta. Histologically, the mass was found to be hilar lymph node with chronic pyogranulomatous inflammation containing organisms characteristic of Blastomyces dermatitidis.