Aims to compare the classification of overweight, obesity and underweight using international v. national approaches in Kuwaiti adolescents. Assessment of underweight, overweight and obesity using a national approach (based on Kuwaiti reference data for BMI-for-age) was compared with assessments obtained using three international approaches: the Cole et al. and International Obesity Task Force (IOTF) definitions of thinness and of overweight and obesity (Cole-IOTF); WHO 2007; and US Centers for Disease Control and Prevention (CDC) 2000 reference data and definitions. The degree of agreement between the different methods was assessed using the weighted κ statistic (κw). Two randomly selected public intermediate schools in Kuwait City. A total of 499 10-14-year-old Kuwaiti adolescents. Prevalence of overweight and obesity using Kuwaiti reference data (36·7 %; 95 % CI 32·4, 41·1) was significantly lower than that obtained using international approaches - Cole-IOTF (44·7 %; 95 % CI 40·3, 49·2), CDC 2000 (44·9 %; 95 % CI 40·5, 49·4) and WHO 2007 (50·5 %; 95 % CI 46·0, 55·0) (P < 0·01). All three international approaches showed almost perfect agreement: IOTF v. WHO (κw = 0·82; 95 % CI 0·79, 0·85) and IOTF v. CDC (κw = 0·90; 95 % CI 0·87, 0·92). However, Kuwaiti reference data showed the lowest agreement with the three international approaches, the poorest being with WHO 2007 (κw = 0·54; 95 % CI 0·49, 0·59). Caution should be exercised when using recently collected national reference data and definitions while assessing underweight, overweight and obesity for clinical and public health applications.