Intervention studies in youth with obesity that can be translated into primary care are limited. We compared a lifestyle intervention to a brief intervention applied by primary care physicians (control group) for treating pediatric obesity in the primary care setting. Seventy-six youth with obesity (body mass index [BMI] >95th percentile or >90th percentile plus waist circumference >90th percentile, aged 9 to 17 years) participated in a 12-month, randomized, controlled trial, conducted at a primary care unit in Northern México from June 2006 through October 2007. Participants randomized to lifestyle intervention attended a family-centered program consisting of 12 sessions of behavioral curriculum, dietary advice from a registered dietitian (weekly for the first 3 months and monthly thereafter), and monthly consultations with a primary care physician. Control group participants attended monthly consultations with a primary care physician who received a brief training on obesity. Forty-three (57%) participants completed the 12 months of study. After 12 months, mean changes (95% confidence interval) in body weight for the lifestyle group and the control group were -0.8 kg (-3.2, 1.5) vs +5.6 kg (3, 8.2; P<0.001) and mean changes in BMI were -1.8 (-2.6, -0.9) vs +0.4 (-0.5, 1.3; P<0.001), respectively. Intention-to-treat analysis at 12 months confirmed significant differences in primary outcomes (weight -3.5 kg, P=0.02; BMI -1.2, P=0.03) in favor of the lifestyle group. This study provides preliminary evidence that primary care physicians supported by a registered dietitian and a behavioral curriculum can be a successful strategy for treating pediatric obesity in the primary care setting. © 2010 American Dietetic Association.