Background: Asthma is the most common chronic childhood disease and has significant impact on morbidity and mortality in children. Proper adherence to asthma medication has been shown to reduce morbidity among those with asthma; however, adherence to medications is known to be low, especially among low-income urban populations. We conducted a randomized clinical trial to examine the effectiveness of an intervention designed to increase adherence to asthma medication among children with asthma that required daily collection of data. Purpose and Methods: A specifically designed web-based data collection system, the Asthma Agents System, was used to collect daily data from participant children at school. These data were utilized to examine the intervention's effectiveness in reducing the frequency of asthma exacerbations. This study examines the Asthma Agents System's effect on the frequency of missing data. Data collection methods are discussed in detail, as well as the processes for retrieving missing data. Results: For the 290 children randomized, 97% of the daily data expected were available. Of the outcome data retrieved via the Asthma Agents System, 5% of those expected were missing during the period examined. Limitations: Challenges encountered in this study include issues regarding the use of technology in urban school settings, transfer of data between study sites, and availability of data during school breaks. Conclusions: Use of the Asthma Agents System resulted in lower rates of missing data than rates reported elsewhere in the literature. © Society for Clinical Trials 2008 SAGE Publications.