There is an evidence to suggest that testosterone is an important regulator of insulin sensitivity in men via effects on body fat distribution and insulin action in muscle. Observational studies have shown that metabolic syndrome is strongly associated with low testosterone levels in men and hypogonadism is likely a fundamental component of metabolic syndrome. Low testosterone could be not only an early marker of metabolic syndrome but also involved in the pathogenesis of disease processes from the prospective studies. Although obesity, insulin resistance and hypogonadism are certainly thought to be linked, the exact causal relationships among them remain unclear. Large population based prospective observational and intervention studies are needed to elucidate the exact causal relationship among testosterone and the components of metabolic syndrome. (Korean J Med 73:553-555, 2007)