Alcohol consumption synergistically increases the risk and severity of liver damage in obese patients. To gain insight into cellular or molecular mechanisms underlying the development of fatty liver caused by ethanol-obesity synergism, we have carried out animal experiments that examine the effects of ethanol administration in genetically obese mice. Lean wild-type (WT) and obese (ob/ob) mice were subjected to ethanol feeding for 4 wk using a modified Lieber-DeCarli diet. After ethanol feeding, the ob/ob mice displayed much more pronounced changes in terms of liver steatosis and elevated plasma levels of alanine aminotransferase and aspartate aminotransferase, indicators of liver injury, compared with control mice. Mechanistic studies showed that ethanol feeding augmented the impairment of hepatic sirtuin 1 (SIRT1)-AMP-activated kinase (AMPK) signaling in the ob/ob mice. Moreover, the impairment of SIRT1-AMPK signaling was closely associated with altered hepatic functional activity of peroxisome proliferator-activated receptor γ coactivator-α and lipin-1, two vital downstream lipid regulators, which ultimately contributed to aggravated fatty liver observed in ethanol-fed ob/ob mice. Taken together, our novel findings suggest that ethanol administration to obese mice exacerbates fatty liver via impairment of the hepatic lipid metabolism pathways mediated largely by a central signaling system, the SIRT1-AMPK axis.