The effects of dietary Yamabushitake mushroom (Hericium erinaceus) on lipid metabolism were examined. C57BL/6J mice were fed a high-fat diet containing hot-water extract (HW-E) and an ethanol extract (EtOH-E) of Yamabushitake mushroom. Administration of HW-E or EtOH-E with a high-fat diet for 28 d resulted in a significant decrease in body weight gain, fat weight, and serum and hepatic triacylglycerol levels. Our in vitro experiments indicated that EtOH-E acts as an agonist of peroxisome proliferator-activated receptor α (PPARα). Quantitative analyses of hepatic mRNA levels revealed that EtOH-E administration resulted in up-regulation of mRNA for a number of PPARα-regulating genes in spite of the fact that the gene expression of PPARα did not change. These results suggest that EtOH-E improves lipid metabolism in mice fed a high-fat diet, and that these effects were mediated by modulation of lipid metabolic gene expression, at least in part via activation of PPARα.