Objectives The current study aimed to identify factors that could predict attrition in youths starting ambulatory treatment to control or lose weight. Design Retrospective longitudinal study. Setting Paediatric clinic: ambulatory treatment programme. Patients and measures A youth sample (n=191; 89 boys; aged 7–17 years) completed measures of demographic characteristics, and health and psychosocial traits before starting an ambulatory weight management programme. Anthropometric and biological markers related to obesity were also obtained. Tests of mean differences and regression analyses were used to investigate the relationship between these variables and attrition after 1 year. Results The χ 2 and t test results showed both psychosocial and health indicators differentiated between participants who continued attending the treatment programme and those who dropped out. More specifically, youths that dropped out of treatment were significantly older, had higher body mass index z scores, higher levels of insulin, triglycerides and HOMA-IR, reported poorer health, had more conduct problems and were more dissatisfied with themselves and their bodies before starting treatment. Results of regression analyses revealed that weight status (anthropometric and biological markers), age and body dissatisfaction predicted attrition (overall prediction success 73%; prediction success for continued attendance 90/91%; prediction success for dropouts 42/44%). Conclusion Attrition, but especially the continued attendance in treatment, can be successfully predicted by age, weight status and body dissatisfaction. For patients who present with one or more risk factors, careful consideration is needed to decide which (combination of) inpatient or outpatient programme may facilitate prolonged engagement of the patient and hence may be most effective in establishing weight loss.