Objective: Maintaining weight loss results in childhood obesity treatment is difficult to achieve. Self-management techniques such as self-monitoring are associated with increased weight loss and maintenance. This study analyzes whether self-monitoring of lifestyle behaviours through a short message service maintenance treatment (SMSMT) via mobile phones with personalized feedback positively effects weight, lifestyle behaviours and psychological well-being in obese children. Methods: After 3 months of behavioural lifestyle treatment, 141 overweight and obese children (7-12 years) were randomly assigned to an intervention group receiving SMSMT for 9 months (n = 73) or to the control group (n = 68). The intervention group sent weekly self-monitoring data on exercise and eating behaviour and their mood via mobile phones. In return, they received tailored feedback messages. Primary treatment outcomes were weight, eating behaviour and psychological well-being, i.e. competence, self-esteem and quality of life. Secondary outcome was adherence to the SMSMT. Data were analyzed with mixed modelling. Results: SMSMT did not improve treatment outcomes. Controls gained temporarily in physical health scores (P = 0.01). SMSMT completers sent on average every 2 weeks an SMS. Children who had greater weight loss during the first 3 months of lifestyle treatment sent more SMSs (P = 0.04). Conclusions: We did not find a positive effect of SMSMT on weight, eating behaviour or psychological well-being in obese children. SMSMT seems to be a feasible method of treatment delivery. Future research should study variations of SMSMT to investigate how SMSMT can be more effective. © 2012 The Authors Pediatric Obesity © 2012 International Association for the Study of Obesity.