BACKGROUND: Obesity is defined as an excess accumulation of body fat. To measure body fat accurately is difficult, but body mass index (BMI, kg/m2) is easily available for routine clinical use. METHODS: A total of 4,966 (male; 2,580, female; 2,386) patients from January to September in 1997 and aged up to 90 years old were studied for BMI. Among these subjects, obstetric patients were excluded in our study. We calculated the BMI as weight (kg) per height squared (m2). The heights and weights of study subjects were obtained from anesthesia records. The BMI was analyzed by 4 different age groups: group 1 (less than 3 years), group 2 (between 3 to 7 years), group 3 (between 8 to 17 years), and group 4 (18 years or more), decade of age, 4 obesity categories according to BMI levels: underweight (less than 18 kg/m2), normal weight (19-24.9 kg/m2), overweight (25-29.9 kg/m2), and obese (30 kg/m2 or more), and surgical departments. RESULTS: Mean BMI values of group 1, 2, 3, and 4 were 16.1+/-2.9 kg/m2, 16.2+/-2.4 kg/m2, 19.2+/-3.7 kg/m2 and 23.3+/-4.6 kg/m2 in males and 15.7+/-3.9 kg/m2, 15.8+/-2.7 kg/m2, 19.4+/-3.5 kg/m2 and 23.6+/-3.7 kg/m2 in females, respectively. According to generations, the BMI under teen-age was 16.3+/-2.7 kg/m2 and 14.3+/-2.9 kg/m2 in males and females respectively while it was between 20.4+/-3.3 kg/m2 to 25.0+/-3.6 kg/m2 from 1st to 7th decade patients in both sex. The prevalence of overweight and obesity were nearly zero in group 1 and 2, but males in group 3 had them of 4.6% and 1.4% while 6.9% and 0.8% in females, respectively. In group 4 they were 25.3% and 2.2% in males and 28.8% and 4.8% in females, respectively. Obese patients increased in the order of neurosurgery, gynecology, orthopedic surgery, urology, and general surgery departments. CONCLUSIONS: This study revealed the guidelines of BMI of operating patients. We recognized that the prevalence of overweight and obesity of preschool aged and adolescent patients were nearly zero. Adult patients were the highest at 4th decade in males and 6th decade in females and the obesity rates were 27.4% and 33.6% in males and females, respectively.