Hepatic lipidosis is the most common liver disease of cats in North America. Unfortunately, clinical examination cannot readily distinguish between cats with hepatic lipidosis and cats with other liver diseases. Since treatments for these diseases can vary markedly, the importance of an accurate diagnosis cannot be overemphasized. Currently, a definitive diagnosis of hepatic lipidosis requires a liver biopsy. However, a biopsy is an invasive procedure that can result in life threatening blood loss. Clearly, a noninvasive test would be preferable and substantially reduce diagnostic risk. Leptin is a hormone primarily produced by fat cells. The rate of leptin secretion and its plasma concentration are correlated with the amount of fatty tissue. In people, leptin concentrations have been shown to be elevated in patients with hepatic lipidosis. We propose that leptin concentrations may be significantly increased in cats with hepatic lipidosis and that they may serve as a useful diagnostic marker. We will compare leptin concentrations in cats that are clinically normal, cats with hepatic lipidosis, and cats with liver disease from other causes.