Background: Ketosis is an important metabolic disorder of dairy cows during the transition period. There have been many reports on the etiology of ketosis in periparturient cows, but little is known about its molecular etiology.Objectives: The objective of this study was to clarify the status of fat mobilization and mRNA abundance of leptin and hormone-sensitive lipase in cows with spontaneous clinical ketosis.Animals: Ten ketotic Holstein cows and 10 nonketotic Holstein cows were used as the experimental animals. Methods: Six blood biochemical parameters were evaluated by means of individual analysis method for 2 groups of cows. The mRNA abundance of leptin and hormone-sensitive lipase in tail fat tissue from 2 groups of cows was measured by real-time (RT)-PCR, with a fluorescent Taqman probe and a standard curve.Results: The plasma concentrations of glucose (P = 0.01), and leptin (P = 0.03), insulin (P = 0.05), and the ratio of insulin to glucagon (P = 0.04) were lower in ketotic compared with nonketotic cows, whereas there were marked increases in the plasma concentrations of nonesterified fatty acid and b-hydroxybutyric acid (P = 0.005). The mRNA abundance of leptin (P = 0.04) and hormone-sensitive lipase (P = 0.02) in the fat tissue of ketotic cows was lower relative to that of nonketotic cows.Conclusions and Clinical Importance: The ketotic cows showed characteristics of type I ketosis and some adaptive changes to negative energy balance in the plasma leptin concentration and mRNA abundance of fat leptin and hormonesensitive lipase.