OBJECTIVE: Quantify Crohn's disease (CD) patients' maximum acceptable risk (MAR) of treatment-related serious adverse events (SAE) for improvements in daily symptom experience. METHODS: An on-line panel of self-identified CD patients completed a series of choice-format conjoint tradeoff tasks. Patients selected between treatment alternatives with varying efficacy and risk levels. The treatment attributes included the daily symptom severity and activity limitations, the potential for serious complications (fistulas, abscesses, bowel obstructions), the time between flare-ups, oral steroid use and varying levels of SAE risks, including progressive multifocal encephalopathy (PML) death or disability, and death from serious infection or lymphoma. The questionnaire also contained items regarding demographics, disease and treatment history, and the short form of the Inflammatory Bowel Disease Questionnaire (SIBDQ). The maximum acceptable 10-year MAR was calculated for various clinical benefit levels. RESULTS: A total of 357 patients completed the survey. Improvements in daily symptom severity were the most important factor in treatment preferences. Higher MAR (greater risk acceptance) was observed for trade-off tasks involving higher levels of clinical benefit, patients with lower SIBDQ scores, and patients reporting a low level of worry about SAEs. For the PML SAE, mean MARs (SD) for an improvement from severe daily symptoms to remission and moderate daily symptoms to remission were 7.2% (0.26) and 4.7% (0.18), respectively. The lowest observed MAR for any of the three studied SAEs is well above the observed rates of SAE occurrence with natalizumab or any of the commonly used CD medications. CONCLUSIONS: Medical interventions carry risks of adverse outcomes that must be evaluated against their clinical benefits. CD patients indicated they are willing to accept a defined risk of death or disability in exchange for clinical efficacy. The patient perspective on the balance between potential benefits and risks can assist in making treatment and regulatory decisions.