Background: Childhood and adolescent obesity constitute a significant public health concern. Family health care settings with multidisciplinary teams provide an opportunity for weight loss treatment. The objective of this study was to examine the effect of intensive treatment designed to reduce weight using a parent-child lifestyle modification intervention in a family health care clinic for obese and overweight children who had failed previous treatment attempts.Methods: This was a practice-based 6-month intervention at Maccabi Health Care Services, an Israeli health maintenance organization, consisting of parental education, individual child consultation, and physical activity classes. We included in the intervention 100 obese or overweight children aged 5 to 14 years and their parents and 943 comparison children and their parents. Changes in body mass index z-scores, adjusted for socioeconomic status, were analyzed, with a follow-up at 14 months and a delayed follow-up at an average of 46.7 months.Results: The mean z-score after the intervention was lower in the intervention group compared to the comparison group (1.74 and 1.95, respectively; P ؍ .019). The intervention group sustained the reduction in z-score after an average of 46.7 months (P < .001). Of the overweight or obese children, 13% became normal weight after the intervention, compared with 4% of the comparison children.