Because the results of the nutritional status released by Chilean institutions which collect anthropometric data on their population under control, show different increments in the prevalence of obesity between preschoolers and children in 1st grade, we decided to verify the real magnitude of this increase. This study is based on a longitudinal investigation which began in 2006 and included 1100, three-year old children, which we evaluated when they were 4, 5 and 6 y. The nutritional status was determined according to BMI Z and WHO References 2006/2007. The information was also used to determine the concordance between the anthropometric data collected on first graders by teachers from public schools in 2009 (JUNAEB data set) and the same children assessed in parallel by INTA (INTA data set). The sample included 474, six-year olds. No differences were found between the average BMI Z and HAZ of the two data sets, and the degree of concordance between the BMI Z's was good (rho = 0.73), however there were differences in the nutritional status, since the prevalence of low weight was significantly greater when assessed with JUNAEB's data (9.5% vs. 3.6% ), while that for obesity was lower with INTA's data, 17.5% vs. 19.2% (not significantly different). At 4 and 5 y, obesity prevalence of the children was 13.3% and 15.7% respectively (INTA's data). Considering the observed nutritional trajectory, it is likely that the rise in obesity between 4 and 6 y of age, was 4 percentage points and not 6, as was determined with INTA's data.