This study examined adherence to angiotensin-converting enzyme inhibitor therapy among 171 heart failure clinic patients. Adherence was monitored over a 3-month period with an electronic event monitor housed in a medication bottle cap, which recorded the date and time the cap was opened and closed. The average percentage of days that the prescribed number of doses (regimen adherence) was taken over the observation period was 84%. Seventy-one percent of patients showed 85%–100% adherence with their daily regimen; 19% exhibited less than 70% adherence. The overall high rates of adherence to angiotensin-converting enzyme inhibitor therapy observed among heart failure clinic patients is consistent with research that shows improved outcomes for patients managed in heart failure clinics. Electronic medication monitoring can be useful in identifying a substantial fraction of patients who are poorly adherent so that interventions to improve adherence can be targeted toward them. Additional research is needed to develop and test adherenceenhancing interventions.