BACKGROUND AND OBJECTIVES: Compliance to a prescribed antihypertensive regimen influences the management of hypertension in various steps, but studies on this issue are very rare in Korea. The medication event monitoring system (MEMS) is the gold standard in measurement of compliance, which is a special pill container, designed to electronically monitor drug intake patterns. Here, the authors investigated the compliance to the antihypertensive regimen in the cardiology practice of a tertiary care hospital using the MEMS. SUBJECTS AND METHODS: Monitoring using the MEMS was performed in 80 hypertensive patients during monotherapy. Demographic, clinical and psychological profiles were collected through a standardized questionnaire. The parameters used for compliance were the percentage of doses taken (PDT) and the percentage of doses taken correctly (PDTc), according to the prescribed regimen. RESULTS: The mean age of the patients and duration of monitoring were 53+/-10 years and 60+/-26 days, respectively. The median and range for the PDT and PDTc were 97% (88-100%) and 92% (80-97%), respectively. About 16% of patients showed relatively poor compliance (PDT<80%). Predicting factors for poor compliance were a recent history of self-discontinuation of drug treatment, not currently being on medication and a young age (p<0.05). Agreement between intuitive prediction by the physician and the actual compliance was closer to what would be expected by chance (kappa coefficient=-0.11). CONCLUSION: Although average compliance to the single drug antihypertensive regimen was relatively high in the cardiology practice of the tertiary care hospital in this study, a significant proportion of patients show low compliance. Special consideration should be given to patients with factors predictive of poor compliance. Investigation of compliances in other clinical settings is also warranted.