objective: in an attempt to simplify the screening process for detecting obesity in adolescence, the performance of different cutoff values for body mass index (bmi) was assessed in a population-based cohort in southern brazil. methods: a total of 493 adolescents aged 15-16 years who lived in the city of pelotas, brazil, were studied. obesity was defined according to the who criteria taking into account age and sex (a bmi equal to or greater than the 85th percentile of the nhanes i reference, plus subscapular and triceps skinfold equal to or greater than the 90th percentile of the same reference). different bmi cutoff values were used to assess their specificity and sensitivity. results: for boys, bmi325 kg/m2 showed the best performance for detecting obesity, with a sensitivity of 90% and only 5% of false positives. the brazilian proposed criteria that was used had 100% sensitivity but up to 23% of false positives. higher cutoff values were also tested, but there was a slight increase in specificity, accompanied by a marked reduction in sensitivity. conclusions: the bmi cutoff of 25 kg/m2 presented the best performance for screening obesity in the studied sample, and it is recommended for adolescents aged 15 and more in populations with similar characteristics. it provides a single cutoff value to be used in primary health services, eliminating the need for age and sex-specific values and skinfold measurements, and it is also consistent with the cutoff value proposed to identifying overweight adults.