The objective of this study was to determine whether a generalizable best-practice individualized behavioral intervention reduced BMI z score relative to standard dietetic care among overweight children. The design consisted of an assessor-blinded, randomized, controlled trial involving 134 overweight children (59 boys, 75 girls; BMI ≥ 98th centile relative to United Kingdom 1990 reference data for children aged 5–11 years) who were randomly assigned to a best-practice behavioral program (intervention) or standard care (control). The intervention used family-centered counseling and behavioral strategies to modify diet, physical activity, and sedentary behavior. BMI z score, weight, objectively measured physical activity and sedentary behavior, fat distribution, quality of life, and height z score were recorded at baseline and at 6 and 12 months. The intervention had no significant effect relative to standard care on BMI z score from baseline to 6 months and 12 months. BMI z score decreased significantly in both groups from baseline to 6 and 12 months. For those who complied with treatment, there was a significantly smaller weight increase in those in the intervention group compared with control subjects from baseline to 6 months. There were significant between-group differences in favor of the intervention for changes in total physical activity, percentage of time spent in sedentary behavior, and light-intensity physical activity. A generalizable, best-practice individualized behavioral intervention had modest benefits on objectively measured physical activity and sedentary behavior but no significant effect on BMI z score compared with standard care among overweight children. The modest magnitude of the benefits observed perhaps argues for a longer-term and more intense intervention, although such treatments may not be realistic for many health care systems.