Upward trend of growth and overweight has been reported from developed countries. As Thailand has been undergoing rapid economic transition over the previous decade, we conducted an analysis to demonstrate the secular changes of growth over five years follow-up. Subjects were a cohort of 2,252 primary school children recruited in 1992 for the Hat Yai childhood obesity study. Baseline demographic and family data were collected by a questionnaire completed by parents. To quantify the cohort effect, a generalized estimating equations analysis for a cross sectional time series data was undertaken with weight, height, and body mass index (BMI, kg/m2) as a dependent variable and containing a quadratic term of age, sex, year of birth and family variables. One thousand and ninety-four (48.5%) children completed 6 anthropometric measurements. Graphs of median weight, height, BMI and overweight prevalence of each birth cohort against age showed secular increases of growth and overweight, and an age effect. For each one-year younger cohort, the median weight, median height and median BMI increased by 1.22 kg, 1.25 cm and 0.23 kg/m2, respectively. An increasing trend of childhood overweight reported here may signify a need for preventing overweight and reducing weight in childhood adolescence in the future.