Rationale: We developed an evidence-based decision aid for patients with advanced cystic fibrosis considering referral for lung transplantation. Objectives: To prospectively evaluate whether use of the decision aid increased knowledge about the options, improved realistic expectations, and decreased decisional conflict in adult patients. Methods: We performed a single-blind randomized controlled trial involving 149 adult patients with cystic fibrosis with an FEV1 ≤ 40% predicted from 14 Canadian and Australian centers. All participants received an education and counseling session from their cystic fibrosis team and were then randomized to receive the decision aid or usual care. The decision aid is available online at http://decisionaid.ohri. ca/decaids.html. Measurements and Main Results: The primary end points measured were participants' knowledge, realistic expectations, and decisional conflict evaluated 3 weeks after randomization. Patients randomized to the decision aid had greater knowledge about their options (P < 0.0001) and had more realistic expectations about the benefits and risks of lung transplantation (P < 0.0001) compared with those randomized to usual care. The total decisional conflict score was significantly lower in the decision aid group 3 weeks postrandomization compared with the usual care group (11.6 vs. 20.4; P = 0.0007). Decisions were durable; 88% of patients in the decision aid group and 75% in the usual care group maintained the same choice 12 months after randomization (P = 0.06). Conclusions: Use of a decision aid for patients with cystic fibrosis considering referral for lung transplantation, in addition to usual education and counseling, improves patient knowledge, realistic expectations, decisional conflict, and patient satisfaction. Clinical trial registered with www.clinicaltrials.gov (NCT00345449).