Few interventions have focused on very young children for obesity prevention. This study evaluated a pilot intervention to improve the nutrition and physical activity environments of child care centers serving infants and toddlers. This randomized controlled trial took place in 32 centers in Boston, Massachusetts. The intervention aimed to improve policies and practices related to nutrition and physical activity within the center. For the outcome, observers assessed center environments using the Environment and Policy Assessment and Observation (EPAO) instrument (range 0-320 points) at baseline and the 6-month follow-up. We fit linear regression models with change in EPAO score from baseline to follow-up, controlling for potential confounders for total score, nutrition sub-score, and physical activity sub-score. Intervention centers had a mean (SD) of 98.2 (144.8) children enrolled, while control centers had 59.2 (34.5). In intervention centers, 47.5% of children were white, compared to 46.2% in controls. Fewer intervention centers had outdoor play areas on site (75 vs. 100%) but more had indoor play space (67 vs. 25%). At baseline, intervention centers had a mean (SD) EPAO score of 134.5 (7.0) points and controls had 146.8 (4.8) points. Compared with controls, intervention centers improved their EPAO scores at follow-up by 18.5 points (95% CI 0.1, 37.0; p = 0.049), chiefly through greater improvement in physical activity (12.2; 95% CI -1.6, 26.0; p = 0.075) and not nutrition (6.4; 95% CI -7.1, 19.8; p = 0.385). The pilot showed promise as an intervention to improve center environments, but future studies should include child-level outcomes.