IntroductionOverweightness and obesity are major concerns for the global companion animal population, as is the case with humans. The American Veterinary Medical Association reported that about 33,000 veterinarians were responsible for diagnosing approximately 840 overweight or obese cats and dogs in their respective animal hospitals in 1998 (1). In addition, 52% of 136 cats recruited by houseto-house survey in the United Kingdom (2) and 40% of vet-visiting cats in Glasgow (3) were deemed to be overweight/obese. The latest data in the United States indicate that 19%-29% and 6%-8% of cats are overweight and obese, respectively (4).Many clinical veterinarians in Japan also feel that feline obesity is increasing in prevalence within Japan. However, with very little data or none regarding healthy domestic cats being readily available in Japan, veterinarians are currently unsure as to what real rate of obesity prevalence exists. One reason for this is because owners do not feel the necessity to bring in their overweight cats to the animal hospital without any apparent symptom(s) of disease. Furthermore, compounding the problem is the fact that owners may regard overweight or obese cats as being physically appealing and cute, without truly seeing the potential health risk associated with overweight/ obesity to their animals. For example, obesity can induce a low-grade inflammatory response (5), and accumulated excess fat can result in inadequate secretion of adipokines and dysfunction in metabolic organs. In addition, feline obesity increases the risks of diabetes mellitus, hepatic lipidosis, neoplasia, urinary tract disease, oral cavity and gastrointestinal disease, dermatoses, and lameness (6). Obesity-induced dyslipidemia in cats has the same health consequences as in humans.As such, the objectives of this study are twofold. First, we sought to determine the rate of overweight/obese cats, as determined via body condition score (BCS), in Japan, based accordingly on a cross-sectional pool of healthy cats that were presented to 18 different veterinary clinics across Japan. Second, using a uniform method of biochemical measurement for cat plasma collected all over Japan, we sought to determine plasma biochemical parameter reference values from healthy indoor clientowned cats, using the same cross-sectional pool of healthy cats. In doing so, we also investigated whether age, BCS, and sex could significantly influence changes in plasma Abstract: Many veterinarians feel that feline obesity is increasing in prevalence within Japan, but are unsure as to what real rate of obesity prevalence exists currently. The objectives of this study are twofold. First, we sought to determine the ratio of overweight/obese cats, as determined via body condition score (BCS), based on a cross-sectional pool of healthy cats. Second, using a uniform method of biochemical measurement for cat plasma collected all over Japan, we sought to construct cross-sectional working reference ranges of plasma metabolites using age, BCS, and sex as classifyin...