Background: Health professionals are looking to nutrition-based youth health interventions in K-12 schools to combat the growing obesity crisis; however, none have explored the influences of interventions guided by constructivist learning theory. Purpose: This study examined the influences of a constructivist-oriented nutrition education program on urban middle school students' nutrition knowledge, self-efficacy and behaviors. Methods: A quasi experimental design examined changes in middle schools students' (N = 1,476) nutrition knowledge, self-efficacy and behaviors, relative to a control group (N = 656), in response to a 6-lesson nutrition education intervention. Results: For dietary knowledge and self-efficacy, there were significant group and time main effects and group x time interactions. In addition, there were significant group and time main effects and a group x time interaction for the dietary behaviors related to consuming fruits, vegetables, meats and "other" food groups, but not dairy or grains. Discussion: The constructivist-oriented professional development, curriculum and instruction yielded significant changes in middle schools students' nutrition knowledge, self-efficacy and behaviors. Translation to Health Education Practice: Given the efficacy of the intervention curriculum and instruction, K-12 teachers are encouraged to implement similar high quality, theoretically grounded efforts. However, recommendations are made that additional focus be given to key items that proved resistant to change. (Contains 5 tables.)