STUDY OBJECTIVE: Adherence with controller therapy poses a major challenge to the effective management of persistent asthma. The aim of this study was to explore the patient-related aspects of adherence among adult asthmatics. DESIGN AND PARTICIPANTS: The participants (n = 509 adult asthmatics), recruited from all parts of Denmark, answered the questionnaire concerning asthma knowledge, attitudes, adherence, and treatment through the Internet. RESULTS: A total of 67% of the patients were prescribed inhaled corticosteroids (ICS). However, according to Global Initiative for Asthma (GINA)-guidelines' symptom severity classification, 85% should have been on ICS. Accidental and intentional non-adherence with ICS at least twice a week was reported by 27% and 24%, respectively. In case of deterioration, 60% of the patients preferred to take more reliever medication, instead of increasing the ICS dose. Having a fixed daily routine with regard to medication and following the advice given by their doctor were the main reasons for adherence, whereas lack of perceived symptoms was the main reason for non-adherence. Non-adherence was associated with increasing disagreement with the statements that controller therapy is effective (p < 0.04), as well as an essential part of asthma therapy (p < 0.002). CONCLUSIONS: Both accidental and intentional non-adherence with controller therapy is common among adult asthmatics. The reasons for suboptimal adherence seem to be accessible through education of both patients and caregivers.