Metabolic syndrome is characterized by insulin insensitivity, central obesity dyslipidemia, and hypertension. It is recognized as a risk factor for cardiovascular disease in men; by the time metabolic syndrome is diagnosed, however, most men already have entrenched cardiovascular disease. A reliable early warning sign is needed to alert physicians to those at risk for metabolic syndrome and cardiovascular disease. Low serum testosterone level has emerged as a reliable prognosticator of metabolic syndrome in men whose testosterone deficiency is genetic (Klinefelter syndrome), iatrogenic following surgery for testicular cancer, pharmacologically induced by gonadotropin-releasing hormone during prostate cancer treatment, or a natural consequence of aging. One third of men with type 2 diabetes mellitus are now recognized as testosterone deficient. Emerging evidence suggests that testosterone therapy may be able to reverse some aspects of metabolic syndrome.