Liver fibrosis is a morphologic alteration that accompanies chronic liver diseases. Apart from analysis of liver biopsy specimens, there has been no means of diagnosing and evaluating the course of liver fibrosis in the dog. Several plasma markers, including transforming growth factor beta-1 (TGF-beta1), are used to indicate liver fibrosis in humans, but none has been validated for use in dogs. There is a significant correlation between the presence and severity of hepatic fibrosis and the plasma concentration of TGF-beta1 in humans with hepatic fibrosis and cirrhosis. The feasibility of using TGF-beta1 as a marker for hepatic fibrosis in dogs was evaluated by comparing plasma concentrations in 29 healthy dogs and 18 dogs with liver disease. The plasma concentrations of TGF-beta1, were 193 to 598 pg/mL in the healthy dogs, 143 to 475 pg/mL in the 7 dogs with mild hepatic fibrosis or none at all, and 427 to 1289 pg/mL in 11 dogs with moderate to severe hepatic fibrosis. The plasma concentrations of TGF-beta1 in the dogs with moderate to severe fibrosis differed significantly (P < 0.001) from those in the other 2 groups, whereas the concentrations in the dogs with mild or no fibrosis did not differ significantly from those in the healthy dogs (P > 0.05). It was concluded that TGF-beta1 is a potential plasma marker for hepatic fibrosis in dogs.