To assess the prevalence of obesity, overweight (including obesity) and thinness in children of the city of Florianopolis (southern Brazil). Cross-sectional study. Representative sample of 7-10-y-old schoolchildren of the first four grades of elementary schools (1432 girls, 1504 boys). Measurements of weight, height and triceps skinfold thickness (TSF) were taken following standard techniques. The body mass index (BMI) was computed as weight/height2. Nutritional status was defined using two references: (1) the Must et al reference for BMI and TSF to define thinness, overweight and obesity (5th, 85th and 95th percentiles, respectively); (2) the International Obesity Task Force (IOTF) BMI cutoffs to define overweight and obesity. Using BMI, according to the Must et al, and IOTF references, the prevalence of obesity was 10.6 and 5.5%, respectively; overweight (including obesity) affected 26.2 and 22.1% of children, respectively. According to the Must et al reference, the prevalence of thinness was 3.2%. Using TSF rather than BMI, according to the Must et al references, fewer children were classified as obese (8.0%) or overweight (20.2%) and more children were classified as thin (4.9%). This study supports the previously reported high frequencies of childhood overweight and obesity in developing countries. The data allow comparisons with other studies carried out in Brazil and other parts of the world.