Objective: To describe the prevalence of 'graded thinness' in children aged 11, 13 and 15 years in eleven developed countries and to identify trends in the prevalence of 'thinness' (BMI, 17 kg/m(2) at age 18 years) by age and gender.Design: Cross-sectional study using data collected through self-reported questionnaires. Setting: Data were taken from the 1997/1998, 2001/2002 and 2005/2006 surveys of the Health Behaviours in School-aged Children (HBSC) Study.Subjects: Children and adolescents from ten European countries and the USA (n 158 000).Results: Prevalence of grades 1, 2 and 3 of thinness was higher among 11-year-old students compared with the 13-and 15-year-olds in all countries. A higher prevalence of thinness was observed in girls than in boys. Since 1998 the prevalence of thinness decreased steadily in Czech boys and girls, while it increased for French girls. In the total European sample of females, thinness decreased from 1998 to 2006 (chi(2) for trend, P<0.01). Age-adjusted logistic regression analysis showed that Czech boys and girls, and Flemish and American girls were less likely to be thin in 2006 than in 1998; while a noteworthy increment, even if borderline significant, was observed for French girls with a 41% increase in the likelihood to be thin.Conclusions: Our findings suggest that thinness is an important overlooked phenomenon with wide variation in prevalence and trends across developed countries. It deserves further longitudinal studies in a multinational context that could increase the understanding of the factors associated with thinness and contribute to developing preventive and nutritional programmes targeted at controlling obesity and chronic diseases, while monitoring thinness.