Good adherence to inhaled corticosteroid (ICS) therapy is essential for effective asthma control. The factors affecting ICS therapy adherence vary among individuals and countries. As few data on adherence have been reported in Korea, the factors influencing such adherence, and the clinical implications thereof, were evaluated in Korean asthma patients. A total of 185 asthma patients who had taken ICS regularly for over 1 year were randomly selected from the recently established domestic adult asthma cohort, COREA (Cohort for Reality and Evolution of Adult Asthma Korea). To obtain adherence to ICS, both prescription refill adherence and self-reported adherence over 1 year (these are objective and subjective measurements respectively) were assessed without any interventions that might affect patients' adherence to ICS. Patients' information such as age, sex, smoking history and number of medication taken, was collected. Assessment of asthma severity, pulmonary function tests, and asthma symptom score were performed to evaluate the possible clinical implication of adherence to ICS. Approximately half of the patients (50.9%) showed less than 80% of prescription refill adherence. There was a considerable discrepancy between prescription refill adherence and self-reported adherence especially in the patients whose refill adherence was under than 50%. Younger asthma patients showed lower adherence to ICS than did older (> or = 60 years old) patients. Higher asthma severity was significantly associated with lower refill adherence to ICS. However, asthma symptom scores and forced expiratory volume in 1 second (FEV(1)) values were not directly related with refill adherence. To improve asthma control in Korea, enhancement of adherence to ICS is critical: our findings emphasize the need to use objective measurements when adherence to asthma medication is to be assessed in clinical practice.