OBJECTIVES: The effectiveness of 5-aminosalicylic acid (5-ASA) therapy for mild to moderate ulcerative colitis (UC) is commonly affected by poor medication adherence.  The present study was designed to determine if adherence behaviour could be explained by differences in patient preferences for 5-ASA therapies. METHODS: A discrete choice experiment (DCE) survey was used to explore patient preferences for different aspects of oral 5-ASA therapy.   The DCE survey captured trade-offs that patients were willing to make and was based on a literature review, clinician interviews, and in-depth interviews with UC patients (followed by cognitive debriefing).  Six attributes were identified: Ease-of-swallowing, Number of administrations per day, Number of pills (per administration), Symptom flare resolution; Likelihood of flare occurrence and Cost (to estimate willingness-to-pay (WTP) for improvements in attributes).  Adherence behaviour was assessed using the Modified Morisky Scale.   Participants (mild to moderate UC patients, n=400) in the UK, US, Germany, and Canada were recruited through specialist patient recruitment agencies and the survey was administered via the internet, following IRB approval.  Data were analysed using the conditional logit procedure. RESULTS: Clinical effectiveness was most highly valued by participants independent of country of origin (e.g. reduction in annual flare risk to 10%: WTP= £78.81 per month) and a return to normal bowel functioning with mucosal healing (WTP= £29.24).   Significant interaction terms identified that people who reported good adherence placed greater value on symptom control compared with self reported poor adherers to therapy (P=0.011). CONCLUSIONS: Data suggest that the most highly valued aspect of therapy was effectiveness.  Therefore, patients may adhere to a medication better if they place greater value on its ability to effectively treat their UC.  Furthermore, these data suggest a possible avenue for physicians to explore with their patients to improve adherence to the treatment regimen by highlighting the risk-benefit profile of 5-ASAs in the treatment of UC.