[Introduction] Mushroom constituents have been found to be highly effective in the prevention and treatment of lifestyle diseases such as lipidosis, high blood pressure, and diabetes, which are closely linked to eating habits, and several varieties of functional foods have been developed from these constituents. As a result, doctors of Western medicine in particular, who in the past have been dismissive of herbal or Oriental medicine, are now tending to take a more proactive stance toward adopting the better aspects of alternative, complementary or traditional medicine. Natural remedies and folk medicines have been incorporated into the treatment of cancer, for example, and in the same way it has become common to incorporate mushrooms into treatment regimes before and after surgical interventions and alongside therapies such as chemotherapy, radiation therapy, and hormone therapy. Mushroom chitosan, which is investigated in this study, is a novel functional ingredient made from the mushroom Flammulina velutipes (Curt.:Fr.) Sing., which is the most common edible mushroom to be artificially cultivated in Japan and has long been part of the Japanese diet. Unlike chitosans of crustacean origin, mushroom chitosan is rich in the main structural component β-glucan, and this dietary fiber is expected to have positive functions within the body. In particular, there are hopes that mushroom chitosan will inhibit cholesterol and fat absorption in the small intestine, and suppress total cholesterol and neutral fat levels in the serum. Previous human trials have confirmed the anti-metabolic syndrome efficacy of supplements containing mushroom chitosan. Here we report the ameliorative effects of mushroom chitosan in an animal model of genetic obesity and lipidosis. [Methods] Mushroom chitosan (RSK2, Ricom Corporation) was administered at different doses to Crj:(ZUC)-fa/fa rats an animal model of obesity and hyperlipidemia continuously for 10 weeks. The rats were kept at a temperature of 22±1ºC and humidity of 60±10%, and illuminated with fluorescent lamps for 12 h/day (07:00~19:00). Body weight, food consumption, body condition, and hematological and blood biochemical blood parameters were measured, and pathological examination (pathological analysis of hepatic lipid droplets) was performed using HE staining. [Results and discussion] Mushroom chitosan (RSK2) showed high efficacy in suppressing weight gain in Crj:(ZUC)-fa/fa rats presenting obesity due to genetic lipidosis. The smallest effective dose was 3 mg/kg. In addition, values for neutral fat, β-lipoproteins and total lipid due to ingestion improved to the normal values. Moreover, the pathological study of the liver revealed a decrease in lipid droplets appearing in the central zone of the hepatic lobule and a decrease in fat deposition in the liver in the group that ingested mushroom chitosan. The results also suggested that serum lipid levels were improved through egestion of excess fat with the feces. Mushroom chitosan (RSK2) was shown to be effective in controlling increase in serum lipids as it has lipase-inhibiting activity, and was shown to control fat deposition in internal organs through egestion of excess blood lipids with the feces. Mushroom chitosan is thus a functional food that is effective in preventing and treating contemporary lifestyle diseases.