In 2001, the Chilean National Nursery Schools Council Program reduced by 10% the energy content (similar to 418.7 kJ) of meals served to children to reduce obesity. We assessed the impact of this measure on obesity and stunting among beneficiaries 2-5 y old. The energy reduction was staggered over 3 y, allowing for a quasi-experimental design involving early (2001), mid (2002), and late (2003) intervention groups. Routine anthropometric measurements (similar to 64,000/y) taken from 19962005 were obtained from registries; obesity (BMI-for-age Z-score >= 2 SD) and stunting (height-for-age Z-score : 2 SD) were defined using the 2006 growth standards. Segmented regression analyses were conducted by intervention group to contrast pre- and postintervention trends. Overall, obesity was high (15.9%), with levels consistently higher in fall and winter as reported in other studies. Preintervention obesity trends increased in the early group (P = 0.001) but decreased in the late intervention group (P 0,02). The impact of the energy reduction on obesity was inconsistent, with reductions in the early group (P < 0.01) but with no change in mid and late intervention groups (P > 0.05). Stunting prevalence was almost as low as in the growth standard (3.2 vs. 2.3%) and decreased preintervention in all groups (P < 0.05). Stunting prevalence increased postintervention (P < 0.05) in all but the late intervention group, where there was no change. Despite a robust design and the ability to detect small seasonal changes in obesity, our analyses showed that the 10% energy reduction did not consistently decrease obesity. The intervention may have slowed improvements in linear growth, but concern is tempered by the near absence of growth failure. J. Nutr. 138: 2237-2243, 2008.