Our aim was to determine the relationship between body fat percentage (BF%) and body mass index (BMI) and to evaluate the validity of World Health Organisation's BMI cut-off values for obesity. Adult out-patients (n=909, 249 men, 660 women), mean age; 40.5 +/- 14.1 years were included. According to WHO's BMI criteria, 440 subjects were obese (79 men, 361 women). The BF% of participants were measured using a bioelectrical impedance analysis (BIA) system (TANITA). Randomly selected 30 patients were also subjected to the dual-energy X-ray absorptiometry (DEXA) procedure for evaluation of the validity of TANITA measurements. The BF% results obtained by DEXA and TANITA revealed good correlation (r =0.952, p= 0.382). There was a positive correlation between BF% and BMI (p<0.001) for both methods. Cut-off values for BMI were calculated as 28.0 kg/m2 for women, 28.2 kg/m2 for men, if obesity was defined as BF >= 25% in men, >= 35% in women according to WHOfs criteria. Using the new cut-off values, the frequency of obesity increased up to 33.9% in our group. The increase was more pronounced in men (67.1% vs. 26.6%). The WHO cut-off values underestimated the frequency of obesity in this population. Further studies are warranted for different ethnic groups.