['To investigate the critical period for overweight development during early childhood by examining growth trajectory and related sex differences.', 'Using piecewise linear mixed models and logistic regression, we examined the effect of growth trajectory at different periods on overweight at age 4-5 by sex among 13,6\u2009971 regularly followed children (mean: 12.2 times) during 2000-2005 in south China.', 'The high-body mass index (BMI) group (>top tertile of BMI Z score at age of 4-5\u2009years) had faster growth rates of BMI, BMI Z score, weight and height than the low-BMI group in the first 3\u2009months of life. Boys were more likely to be overweight [odds ratio (OR)\u2009=\u20092.0, 95% confidence interval: 1.5-2.7] than girls; the male high-BMI group had higher growth rates during the first 3\u2009months than girls with high-BMI, independently of environmental factors. Those fast grown (in the upper tertile of growth rates in BMI and BMI Z score) in periods 0-3\u2009months had relatively higher OR of at risk of overweight at age of 4-5\u2009years than those in other periods.'] Do impacts of infancy rapid weight gain on 5-year childhood overweight development vary by age and sex in China?