Recent prospective, epidemiological research has demonstrated the power of an increased waist/hip circumference ratio (WHR) to predict both cardiovascular disease (CVD) and non-insulin dependent diabetes mellitus (NIDDM) in men and women. Obesity, defined as an increased total body fat mass, seems to interact synergistically in the development of NIDDM, but not of CVD. Increased WHR with obesity (abdominal obesity) seems to be associated with a cluster of metabolic risk factors, as well as hypertension. This metabolic syndrome is closely linked to visceral fat mass. Increased WHR without obesity may instead be associated with lift style factors such as smoking, alcohol intake, physical inactivity, coagulation abnormalities, psychosocial, psychological and psychiatric factors. Direct observations show, and the risk factor associations further strengthen the assumption, that abdominal (visceral) obesity is more closely associated to NIDDM than CVD, while an increased WHR without obesity may be more closely linked to CVD than NIDDM. It remains to be established to what extent, if any, an increased WHR in lean men, and particularly in lean women, indicates fat distribution. Other components of the WHR measurement might be of more importance in this connection.