Childhood obesity is a major public health problem in industrialized countries. The aim of this study was to estimate the risk of obesity at age 5 based on BMI categories at age 3 and changes in BMI z-score from birth to 3 years of age. In this population-based study BMI data of 5173 children were collected at ages 3 and 5 and were linked to information relative to birth weight. The prevalence of obesity at age 5 was 3.8%. The risk of obesity for children born large for gestational age was 6.5%, while it was 18.6% for children overweight at age 3 and 62% for children who were obese at 3. An increase in BMI z-score from birth to 3 years increases the risk of obesity at age 5 (OR for increase of one standard deviation 2.8%; 95% CI: 2.46-3.20), but adjusting for BMI z-score at age 3, the effect of trajectory disappears (OR 1.08 95% CI: 0.9-1.29). In other words, if one targeted early preventive interventions to 3-year-olds affected by overweight/obesity (only 9.8% of the study cohort), one could possibly address 71% of children potentially affected by obesity at age 5.