Obesity is the most prevalent nutrition-related problem in Western societies. Childhood obesity is rapidly emerging as a global epidemic that will have profound public health consequences, as overweight children become overweight adults. The aim of this study was to investigate the prevalence of overweight and obesity among children and adolescents in the city of Thessaloniki, and evaluate the trends in Greece by comparing our results to those of other cross-sectional studies. Data concerning the height and weight of 2,458 schoolchildren aged 6 to 17 years (1,226 6-10 years, 1,232 11-17 years) of 27 primary and secondary public schools were collected. BMI was calculated from the two measurements. In the analyses, the estimations of the prevalence of overweight and obesity are based on recently established international BMI percentile curves and cut-off points from 2-18 years. To investigate the secular trends in obesity in Greece, data of schoolchildren from four successive surveys were used. In the younger group (6-10 yr), the prevalence of overweight and obesity were 25.3% and 5.6%, while for adolescents (11-17 yr) they were 19.0% and 2.6%, respectively. The prevalence was 25.9% and 5.1% for all males, and 19.1% and 3.2% for all females, respectively. As far as trends are concerned, an increase of BMI was found among males when the results of our survey were compared with those of the previous three. However, the trends for girls are different. An increase was found when the results of our study were compared with 1942. A decrease of BMI at most ages was found when the results of our study were compared with those of the 1982 survey, while an increase was recorded only for younger girls below 13 years compared to the 1984-5 study. This study demonstrates that the prevalence of overweight and obesity among schoolchildren is 22.2% and 4.1%, respectively, and has been increasing in the last decades, especially among boys.