The -112A > C polymorphism (rs10011540) of the gene for uncoupling protein I (UCP1) has been associated with type 2 diabetes mellitus in Japanese individuals. The aim of the present study was to investigate the effects of this polymorphism, as well as the well-known -3826A > G polymorphism (rs1800592), on clinical characteristics of type 2 diabetes. We determined the genotypes of the two polymorphisms in 93 Japanese patients with type 2 diabetes. Intramyocellular lipid content and hepatic lipid content (HLC) were measured by magnetic resonance spectroscopy. No significant differences in age, sex, BMI, or HbA1c level were detected between type 2 diabetic patients with the -112C allele and those without it. However, homeostasis model assessment for insulin resistance (p = 0.0089) and HLC (p = 0.012) was significantly greater in patients with the -112C allele. We did not detect an association of the -3826A > G polymorphism (rs1800592) of UCP1 gene with any measured parameters. These results suggest that insulin resistance caused by the -112C allele influences the susceptibility to type 2 diabetes. (c) 2005 Elsevier Inc. All rights reserved.