Background: We evaluated the impact of a brief primary-care-based intervention, The Maine Youth Overweight Collaborative (MYOC), on BMI (kg/m 2 ) z-score change among participants with obesity (BMI ‡ 95th percentile for age and sex), overweight (BMI ‡ 85th and < 95th percentile), and healthy weight ( ‡ 50th and < 85th percentile).Methods: A quasi-experimental field trial with nine intervention and nine control sites in urban and rural areas of Maine, MYOC focused on improvements in clinical decision support, charting BMI percentile, identifying patients with obesity, appropriate lab tests, and counseling families/patients. Retrospective longitudinal record reviews assessed BMI z-scores preintervention (from 1999 through October 2004) and one postintervention time point (between December 2006 and March 2008). Participants were youth ages 5-18 having two visits before the intervention with weight percentile greater than or equal to 95% (N = 265). Secondary analyses focused on youths who are overweight (N = 215) and healthy weight youth (N = 506).Results: Although the MYOC intervention demonstrated significant provider and office system improvements, we found no significant changes in BMI z-scores in intervention versus control youth pre-to postintervention and significant flattening of upward trends among both intervention and control sites ( p < 0.001).Conclusions: This brief office-based intervention was associated with no significant improvement in BMI z-scores, compared to control sites. An important avenue for obesity prevention and treatment as part of a multisector approach in communities, this type of primary care intervention alone may be unlikely to impact BMI improvement given the limited dosage-an estimated 4-6 minutes for one patient contact.