It is well known that the relative risk of developing type 2 diabetes mellitus increases with increasing body mass index. The presence of not only excessive body fat but also the pattern of fat distribution (central obesity) is important for other metabolic abnormalities included in the metabolic syndrome. The possible mechanisms linking obesity to type 2 diabetes including high concentrations of free fatty acids, altered adipokines expression (low adiponectin levels and high expression of tumor necrosis factor-alpha), and low-grade inflammation are reviewed. Finally, the author refers to the theory that the primary defect is beta-cell failure that leads to diabetes as well as to obesity.