Abstract Background Obesity is a complex multifactorial condition for which the treatment needs to be tailored to the individual patient. Cognitive behavioural therapy (CBT) is the mainstay for most obesity treatments. Obesity has been compared with addictive behaviours in which people show characteristics of dysfunctional impulsivity, which manifest as deficits in inhibitory control. We aimed to examine the effect of CBT on the objective behavioural manifestations of impulsivity during weight loss in obese adolescents. Methods 31 obese adolescents (21 boys, 17 girls) with a mean age of 14·0 years (SD 0·3) and a body-mass index (BMI) of 33·6 kg/m2 (0·9) attending an obesity camp were tested for impulsivity with the CANTAB (Cambridge, UK; Cambridge Cognition) Stop Signal Task. Tasks were undertaken before the CBT intervention, 6 weeks after the intervention (range 3–8 weeks), and at 6 month follow-up. The inclusion criteria were being obese when adjusted for age and sex, having had regular education up to GCSE level (at age 15–16 years), and being aged between 11 and 17 years. We excluded patients with a formal diagnosis of attention deficit hyperactivity disorder or an eating disorder, those receiving antipsychotic drugs, and those with a serious physical disability restricting physical activity. Findings Mean BMI fell from 33·60 kg/m2 (SD 5·0; n=31) before camp to 30·94 kg/m2 (4·5; n=31) at the end of camp (p=0·01). At 6 month follow-up, participants' weight loss had plateaued, with a mean BMI of 31·27 kg/m2 (SD 5·0; n=12, p=0·04). Reduction in impulsivity after the intervention remained when age and length of stay in camp were controlled for with a general linear model (23·9 ms [86·45]; p=0·03). Impulsivity measurements were available in 26 of 31 adolescents at the end of camp. Reasons for loss to follow-up were being unwell (n=1) on the day of data collection and leaving camp for the day (n=4). Loss to follow-up was accounted for in the statistical analysis by use of a complete case analysis of all available covariates. Correlations showed a positive relation between length of stay in camp and reduction in stop signal reaction time (SSRT; r2=0·20, p=0·03), whereas we noted an inverse relation between age and reduction in SSRT (F=7·723, p=0·011). At 6 months, 12 of 31 adolescents were followed up. In these patients, impulsivity scores decreased by a mean of 108·45 ms (SD 95·7; p=0·008) compared with baseline values. At 6 months, general linear regression did not show any significant associations; however, impulsivity scores were weakly associated with a reduction in BMI (r2=0·139; p=0·061). Interpretation Impulsivity is an automatic behaviour pattern, thought to be stable over time. We showed that the stop signal task—a commonly used method to measure impulsivity—can be modified over time. Existing data suggest that impulsive individuals are more likely to be obese, but our data suggest that impulsivity might also be associated with weight loss and its maintenance. Limitations of the study include the before and after design in which participants could become familiar with the SSRT task, which could account for the improved scores after camp. We did not measure the intellectual ability of participants, which could be a confounding factor. The follow-up environments were also uncontrolled. This study further contributes to research of obesity based on an addictive behaviour model and treatment implications. Funding None.