Obesity is a common and serious metabolic disorder in the developed world, which is occasionally accompanied by type II diabetes, atherosclerosis, hypertension and hyperlipidemia. We have found that Mest (Mesoderm specific transcript) / Peg1 (Paternally expressed gene 1) gene expression was markedly enhanced in white adipose tissue of mice with diet-induced and genetically caused obesity/diabetes, but not with streptozotocin-induced diabetes which does not cause obesity. Administration of pioglitazone, a drug for type II diabetes and activator of PPAR, in db/db mice reduced the enhanced expression of Mest mRNA in adipose tissue, concomitant with an increase in body weight and a decrease in the size of adipose cells. Ectopic expression of Mest in 3T3-L1 cells caused increased gene expression of adipose markers, such as PPAR, C/EBP and aP2. In transgenic mice overexpressing Mest in adipose tissue, enhanced expression of the adipose genes was observed. Moreover, adipocytes were markedly enlarged in the transgenic mice. Thus, Mest appears to enlarge adipocytes and could be a novel marker of the size of adipocytes.