The association of several risk factors, obesity, dyslipoproteinemia, hepatic steatosis, insulin resistance and hypertension with Type 2 (non-insulin-dependent) diabetes mellitus and myocardial infarction has long been known and has been termed the "metabolic syndrome". In 1988 Reaven introduced syndrome X as the link between insulin resistance and hypertension. It has been suggested that a critical factor in the association between obesity, Type 2 diabetes and cardiovascular morbidity is the mass of intraabdominal fat. Striking similarities exist between the metabolic syndrome and untreated growth hormone (GH) deficiency in adults. The central findings in both these syndromes are abdominal/visceral obesity and insulin resistance. Other features common to both conditions are premature atherosclerosis and increased mortality from cardiovascular diseases. These similarities indicate that undetectable and low levels of GH may be of importance in the metabolic aberrations observed in both these conditions. Recent investigations have found that abdominal/visceral distribution of adipose tissue is associated with endocrine disturbances including increased activity of the hypothalamic-pituitary-adrenal axis and a blunted secretion of GH and sex steroids. Theoretically, these endocrine perturbations can be a consequence of obesity, but the endocrine aberrations may have causal effects. We studied moderately obese, middle-aged men with a preponderance of abdominal body fat. As a group, they had slight to moderate metabolic changes known to be associated with abdominal/visceral obesity. Nine months of GH treatment reduced their total body fat and resulted in a specific and a marked decrease in both abdominal subcutaneous and visceral adipose tissue. Moreover, insulin sensitivity improved and serum concentrations of total cholesterol and triglyceride decreased. Diastolic blood pressure also decreased. The finding that GH replacement in men with abdominal obesity can diminish the negative metabolic consequences of visceral obesity suggests that low levels of this hormone are of importance for the metabolic aberrations associated with visceral/abdominal obesity.