Obesity occurs less frequently in Japanese than in various other ethnic populations. A person with abnormal glucose tolerance is often found to have one or more of the other cardiovascular disease risk factors, such as obesity, hypertension and hyperlipidemia. This clustering has been labeled as metabolic syndrome (WHO, 1998). It was suggested that Japanese, categorized as having normal weight (BMI of less than 25.0), as defined by the WHO (2000), have an increasing tendency toward metabolic syndrome. Our objective was to analyze metabolic syndrome in "Overweight" with BMI of 23.0-24.9 in Japanese workers, and to assess the suitability for Asians of the Regional Office for the Western Pacific Region of WHO criteria pertaining to obesity (WPRO criteria, 2000). We conducted a cross-sectional study in the workplace setting and investigated the relationship between BMI classification based on WPRO criteria and metabolic syndrome by gender and age group (18-44 yr vs. 45-60 yr). Three hundred seventy-nine men and 432 women Japanese workers participated in this study. BMI were categorized as 20% "Overweight" (23.0-24.9 BMI), 20% "Obese I" (25.0-29.9 BMI) and 2% "Obese II" (over 30.0 BMI), based on WPRO criteria. Graded increases in BMI were positively associated with body fat percentage, waist circumference, hip circumference and waist/hip ratio in both genders and age groups. A progressively increasing BMI category in the elder group aged 45-60 yr in both genders was positively related with parameters constituting metabolic syndrome. Graded increases in BMI classes in elder workers based on WPRO criteria were positively associated with prevalence of metabolic syndrome, and "Overweight" elder women had significantly higher prevalence of metabolic syndrome. The present investigation, based on the increasing risks of "Overweight" with a BMI of 23.0-24.9, suggests that WPRO criteria are suitable for Japanese workers aged over 45 yr.