Objective To study the prevalence and trend of obesity among primary and middle school students in Beijing from 1985 to 2005.Methods Morphological data of students aged 7-18 from the National Physical Fitness and Health Surveillances of Chinese School Students in Beijing in 1985,1995,2000 and 2005 was analyzed with methods of cross-sectional study.The prevalence of obesity was calculated,and its secular trends was characterized.Results The prevalence of obesity among students aged 7-18 in Beijing were 0.65%,3.40%,5.23% and 8.53% respectively in the four surveillance programs with obvious rising trend which had an increase.In 2005,it was indicated that the obesity prevalence of subgroups were significantly different from each other.It was higher for males(11.11%)than females(5.98%),also for students aged 10-12(11.63%).In rural schools(6.60%)it was much 10wer than in urban(10.38%),but the areas with high SES was deemed to have a 10wer prevalence.Conclusion In the last two decades,among primary and middle school students in Beijing,obesity had changed its characteristics from sporadic to epidemic with the features of both developed and developing countries.