Routine necropsy examination of a clinically normal, ten-month-old male beagle dog identified disruption of the dorsal aspect of the right median liver lobe by a multiloculated, pale tan-white, thick-walled mass, with diffuse hypertrophy of the left and caudate liver lobes. The nature of the lesion was investigated using a variety of immunohistochemical and histochemical techniques. Histologically, the lesion was characterized by marked, focally extensive vascular hyperplasia and ectasia with lobular atrophy, periportal fibrosis, and biliary hyperplasia. The endothelium of proliferating vessels was positive for von Willebrands factor, and proliferating bile ducts were positive for pancytokeratin. Based on the gross and histologic appearance, this lesion was identified as a vascular hamartoma with arteriovenous fistula. It is important to note that this spontaneous lesion may occur in the laboratory beagle in the absence of clinical signs.