Hepatoblastoma is the most common primary liver tumor in children resulting in approximately 1 diagnosis per 1 million children less than 15 years of age in Western countries. Some association appears to exist between hepatoblastoma and other human congenital abnormalities such as familial adenomatous polyposis and Beckwith-Wiedmann syndrome. The pathogenesis remains unclear. However, alterations in the Wnt signaling pathway, specifically mutations to the B-catenin gene or the regulation thereof, appear to play a major role. Histological classification systems have varied over the years but presently, most human pathologists classify hepatoblastoma by distinguishing between two types -- epithelial and mixed (epithelial and mesenchymal). Various paraneoplastic syndromes may accompany the tumor. In humans serum a-fetoprotein concentration serves as a tumor marker facilitating diagnosis and aids in monitoring disease progression and recurrence. Reported cases of hepatoblastoma in domestic animals are rare but do include horses, sheep, pigs, alpaca, dogs, mice, and a bull. This report provides the clinicopathologic findings of an 11 month old Thoroughbred filly with hepatoblastoma and erythrocytosis.