The authors carried out a retrospectively study of bodyweights of primary-school children from Grade I to Grade VI in 4 schools from Bangkok, one school from Saraburi and data of school children from a district in Sakolnakorn with a six year follow-up period. In the cohort there were 437 children from Bangkok, 225 children from Saraburi and 633 children from Sakolnakorn. Initial data show that prevalence of obesity at grade I in schools from Bangkok, Saraburi and Sakolnakorn were 16%, 23% and 4%, respectively. However, when these children were in grade VI, the prevalence of obesity increased to 31%, 30% and 9%, respectively. Relative risks of becoming obese at grade VI in children who were overweight and obese at grade I are 3 and 5 fold in Bangkok, 2 and 3 fold in Saraburi, 6 and 12 fold in Sakolnakorn, respectively. The present study shows that prevalence of obesity increased at quite dramatic rates during the primary school period in these study groups.