Objective. Evaluate the effect of a multidisciplinary behavioral childhood/adolescent obesity treatment program on body mass index (BMI). Methods. Height and weight were measured and BMI was calculated at start and completion of a 12-session program in 74 consecutive children/adolescents. Results. Mean age was 12.2 ± 3.2 years and mean baseline BMI z score was 2.4 ± 0.3, corresponding to a BMI percentile of 98.9 ± 1.0%. The majority, 57% (n=42), completed all 12 sessions. Those completing all sessions were younger (11.6 vs. 13.2 years old, p < 0.04) and had a lower baseline BMI z score (2.3 vs. 2.5, p=0.01). Twenty-two (52%) had a significant decrease in BMI z score (p < 0.001), with a mean decrease of 13.8%. Conclusions. Lower attrition for a childhood/adolescent obesity treatment program was associated with younger age and lower baseline BMI z score. Over half of participants that completed an educational multidisciplinary obesity treatment program experienced a decrease in BMI z score, independent of their gender.