The present study was designed to establish adequate criteria for categorizing 'obesity disease' in Japan in relation to obesity-related complications. The subjects were 1,193 Japanese subjects (775 men, 418 women; age: 20-84 years old, body mass index (BMI): 14.9-56.4 kg/m(2)) including subjects undergoing a health examination and obese subjects visiting an obesity clinic. Visceral fat area (VFA) and subcutaneous fat area (SFA) were determined by computed tomography (CT) at the umbilical level. Anthropometric parameters, including BMI, waist circumference (W), waist/hip circumference (W/H), ratio and waist circumference/body height (W/BH) ratio, were measured. Hyperglycemia, dyslipidemia, and hypertension were evaluated as obesity-related complications. The relationship between each parameter and the prevalence of the complications was investigated. The number of complications increased in accordance with BMI and the average value was greater than 1.0 at a BMI of 25. The best combination of the sensitivity and specificity for detecting subjects with multiple risk factors was a BMI of 25. BMI showed a close positive correlation with SFA (r=0.82), even for BMI > or =25 (r=0.77), but had a weaker correlation with VFA (r=0.54). The obese subjects with a BMI > or =25 had no correlation between BMI and VFA because of the wide individual variation of VFA. The number of disorders was greater than 1.0 at 100 cm(2) of VFA and the best combination of the sensitivity and specificity for determining subjects with multiple risk factors was 100 cm(2) of VFA. Between the simple anthropometric values and measurement of VFA, it was proven that W had the closest relationship with VFA in both men (r=0.68) and women (r=0.65). The regression line obtained from simple correlation analyses indicated that the W corresponding to 100 cm(2) of VFA was 84.4 cm in men and 92.5 cm in women. These data suggest that obesity is adequately specified as a BMI > or =25 in Japan where the prevalence and degree of obesity remains mild. It is reasonable to establish the cut-off point of VFA at 100 cm(2) as indicative of the risk of obesity-related disorders and a waist circumference of 85 cm in men and 90 cm in women approximates to this visceral fat mass.