OBJECTIVE: Compare maximum acceptable risk (MAR) of treatment-related serious adverse events (SAE) between adult Crohn's disease (CD) patients and parents of children with CD. Information on benefit-risk tradeoffs may aid clinicians and regulatory agencies in their decision-making. METHODS: An on-line panel of adult CD patients and parents of children with CD completed a series of choice-format conjoint tradeoff tasks. The treatment attributes included daily symptom severity and activity limitations, the potential for serious disease complications, the time between flare-ups, oral steroid use, and varying levels of three SAE mortality risks: serious infection, progressive multifocal leukoencephalopathy (PML) and lymphoma. The annual MAR was calculated for 15 SAE-clinical benefit combinations (3 SAEs X 5 levels of clinical improvement from a severe or moderate CD state). RESULTS: A total of 357 adult patients and 105 parents completed the survey. Improvements in daily symptom severity were the most important factor in treatment preferences. Higher MAR (greater risk acceptance) was observed for tradeoff tasks involving higher levels of clinical benefit. Compared to adult patients, parents were willing to accept a greater SAE risk for alleviation of severe CD symptoms (for 7 of 9 SAE vs. clinical benefit combinations), but were less willing to accept SAE risk for improvement of moderate CD symptoms (for 6 of 6 combinations). CONCLUSIONS: Medical interventions carry risks of adverse outcomes that must be evaluated against their clinical benefits. Adult patients and parents of children with CD indicated they are willing to accept defined mortality risks in exchange for clinical efficacy. While parents were more willing to accept higher SAE risks for improvement of severe CD symptoms, patients were willing to accept higher risks for improvement of moderate CD symptoms.