Lifestyle and disease patterns in Malaysia have changed following rapid economic development. It is important to find out how these changes have affected the nutritional status and health behaviour of the population, especially school children and adolescents. Therefore a survey on school children's and adolescents' health behaviours and perception in Kuala Lumpur was initiated. This paper only reports the observed body mass status of the school children. A total of 3620 school children were selected in this survey using the method of multi-stage sampling. The students were surveyed using pre-tested questionnaires while weight and height were measured by the research team in the field. Using the cut-off of BMI-for-age >or= 95th percentile and <5th percentile for overweight and underweight respectively, there were a total of 7.3% of overweight students and 14.8% of underweight students. When analysed by gender; 7.5% of boys and 7.1% girls were overweight, while 16.2% of the boys and 13.3% of the girls were underweight. The youngest age group (11 years old) had the highest prevalence of underweight as well as overweight. With increasing age, the prevalence of underweight and overweight decreased and more children were in the normal weight range. The overall prevalence of overweight among the three ethnic groups was similar. However the prevalence of underweight was highest among the Indian students (24.9%), followed by Malays (18.9%) and Chinese (9.5%) (P <0.001). The results showed that both the problems of under- and over-nutrition co-exist in the capital city of Malaysia. The promotion of healthy eating and physical activities is required to address the problems of under- and over-nutrition in order to build up a strong and healthy nation in the future.