Our previous studies have shown that medication compliance in children prescribed continuous treatment for asthma is poor, and that an intervention can improve the level of compliance. The present study examined the effects of an intervention on the clinical course of moderately severe asthma. At each of six clinic visits, spirometry was performed, medication compliance was assessed by questionnaire, and the physicians made an overall assessment of asthma severity (Severity Score) and provided a score for asthma control (Control of Asthma Score). Peak expiratory flow rates were measured twice daily for one month prior to each clinic visit, and the coefficient of variation (% CV) was calculated. Subjects received the intervention after at least two visits, and 53 of the 78 recruits completed the study. Following the intervention, % CV, Control of Asthma Score, Severity Score and % compliance improved, showing that better medication compliance was associated with better control of moderately severe asthma.