This study examined whether utilizing an electronic medication monitor (Nebulizer Chronolog) to provide participants with detailed feedback on their metered-dose inhaler (ipratropium bromide or placebo) usage patterns would result in closer adherence to the prescribed regimen of two inhalations three times daily compared to a control group not receiving feedback. Adherence was also measured by canister weighing and self-report. Two-hundred fifty-one consecutive special intervention participants from the University of California, Los Angeles, and Johns Hopkins University centers of a National Heart, Lung, and Blood Institute-sponsored clinical trial were enrolled in this ancillary study. Compared to controls, feedback participants at the 4-month follow-up adhered more closely to the prescribed three sets per day (mean 1.95 vs 1.65) and used the prescribed two actuations in a greater percentage of sets (80 percent vs 60.3 percent). These results indicate that electronic monitoring of metered-dose inhaler use with a Nebulizer Chronolog in a clinical trial not only provides a more accurate assessment of adherence to prescribed inhaler use, but also enhances adherence when participants are given feedback of the monitoring results.