Recent studies have indicated that the waist/hip circumference ratio (WHR), an index of abdominal fat distribution, is a risk factor for cardiovascular disease and diabetes, in parallel with other previously established risk factors. Obesity, without taking fat distribution into account, seems to be associated with WHR in its relationship to the metabolic risk factors for these diseases. The important component of the WHR is probably the mass of visceral fat. This cluster of phenomena constitute what has recently been called the metabolic syndrome or syndrome X. Visceral fat mass is probably increased by a multiple endocrine aberration, where steroid hormones are important. This seems to cause insulin resistance by direct effects on the periphery, which may be amplified by the metabolism of the enlarged visceral adipose tissues.