Objective: The success of treatment for pediatric obesity is variable and often unsatisfactory. This study elucidates the influence of inattention and hyperactivity/impulsivity on short- and long-term weight loss and maintenance after outpatient treatment. Methods: We included 8- to 16-year-old overweight and obese participants treated in 17 multidisciplinary outpatient treatment centers in a nationwide observational study. All treatment centers that reported long-term (1-year) follow-up weight data of at least 60% of the participants were included. At the beginning and end of treatment and at 1 year follow-up weight and height were measured at the center. Inattention and hyperactivity/impulsivity were assessed with the hyperactivity/inattention subscale (HI) of the parent-rated Strengths and Difficulty Questionnaire (SDQ). General linear models were used with the standard deviation scores of the BMI (BMI-SDS) as dependent variable and HI scores as main independent variable adjusting for age, sex, baseline BMI-SDS, and center. Results: 394 participants were included (57% female, age: 11.7± 2.0 years, baseline BMI-SDS 2.32 ±.46 kg/m2). HI scores were significantly associated with short- and long-term BMI-SDS (p < 0.0005), with higher baseline HI scores predicting less weight loss. Conclusions: Our results indicate that inattention and hyperactivity/impulsivity are linearly associated with reduced short- and long-term weight loss. Implications for treatment are discussed.