OBJECTIVES Poor asthma control in children is partly caused by poor adherence to medication. The aim of this study was to investigate the effectiveness and cost-effectiveness of a Real-Time Medication Monitoring system (RTMM) to improve adherence to inhalation corticosteroids. METHODS We performed a multi-center, randomized controlled trial. Included were 209 children (<12 years) with moderate to severe asthma, who had used inhaled corticosteroids (ICS) for at least 3 months. Patients were followed for 12 months. All children received an RTMM device, but only in the intervention group text messages were sent to the parents whose child appeared to forget an inhalation. The effectiveness measures were adherence (percentage of inhalations taken within the correct timeframe), clinically relevant improvement in asthma control score (ACT, ≥3) and asthma-related quality of life (PAQLQ, ≥0.5) at the end of the study. Costs were calculated from a healthcare perspective (including GP, hospital, medication and RTMM device costs) and from a societal perspective (additionally including costs of parents’ absence from work). Uncertainty around the point estimates was assessed using bootstrapping. RESULTS Adherence was 73% in the treatment group and 58% in the control group (difference 15%-pt, 95%-CIL 8.6%-22.0%). Of the RTMM patients, 33% showed clinically relevant improvement in ACT and 22% in PAQLQ. In the control group, these numbers were 37% and 38% respectively (differences not statistically significant). Costs were higher in the intervention group: €825 versus €713, a difference of €112 (95%-CI:-€139- €338) from the healthcare perspective and €1084 versus €845 from a societal perspective (difference 239, 95%-CI: -€84 – €565).The incremental costs per 10% improvement from each perspective were €74 and €157 respectively. CONCLUSIONS RTMM increases inhalation adherence, but there is no evidence of better health outcomes in this patient population within the first year. In these circumstances, this is not a cost-effective intervention.