BACKGROUND: Ideal Body Weight(IBW) is commonly used to determine obesity levels because of its usefulness as target weight. This study was attempted to evaluate the comparative consistency between standards of determining obesity levels using the BMI and IBW. And the authors here provide a definition of IBW and attempt to suggest a new classification of obesity by IBW. METHODS: The test subjects for the research were 24,816 patients who received treatment at the SNUH Health Promotion Center between May 1, 1996 and December 31, 1999. The comparative consistency between standards of determining obesity levels using the BMI and IBW were analyzed calculating the kappa value. RESULTS: The IBW from the Broca method produced a kappa value of 0.668(p<0.001), while the calculation method for IBW recommended by the Japan Obesity Institute produced a kappa value of 0.664(p<0.001). On the other hand, an IBW calculated as 21(kg/m2)X[height(m)]2 (kg) produced the highest value of consistency at a kappa value of 0.911(p<0.001). After calculating IBW as 21(kg/m2)X[height(m)]2 (kg) and comparing the kappa values while adjusting the boundary zone standards, it was found that defining underweight as below 88% of IBW and obesity as above 145% produced the highest value of concurrence at a kappa value of 0.935(p<0.001). CONCLUSION: The validity and confidence with respect to the definition of IBW and the determination method for obesity as proposed by the authors may be confirmed through the methods high level of consistency [kappa value=0.935(p<0.001)] with determination methods using the BMI.