Background: Little is known about the extent of alignment between hematopoietic stem cell transplant (HSCT) patients and their healthcare proxies with respect to advance care planning (ACP). Aim: To determine if a structured three-step process using the letter advance directive (LAD) could (1) allow for the differences in opinion between patient-proxy dyads to surface and (2) help bridge preexisting discordance about specific treatment choices. Design: Blinded to each other, the HSCT patient (LAD-1) and proxy (LAD-2) each completed the LAD (step 1). They unmasked, compared LAD-1 and LAD-2, and discussed their choices (step 2). They completed a final letter directive (LAD-3) by consensus (step 3). Settings/Participants: Convenience sample of eighty dyads (patient and proxy) at a regional HSCT referral center. Results: The mean patient-proxy concordance was 72.9% for the 12 questions in the LAD. Wanting to be pain free at the end of life was the statement with the most amount of agreement (88.75% in LAD-1, 91.25% in LAD-2, and 90% in LAD-3). Patient-proxy dyads had notable discordance related to specific treatments. The highest discordance was related to ventilator support (46.3% of patients refused it, while 58.8% of proxies refused on behalf of the patient). Overall, proxies were more likely than patients to opt in for dialyses and hospice care but more likely to opt out for cardiac resuscitation and sedation to palliate refractory symptoms. On open discussion, patient-proxy discordance mostly resolved in favor of the patient. Conclusions: The ACP process should allow for patient-proxy differences to surface, facilitate a discussion about the granular details with the goal of reaching consensus. Our three-step approach using the LAD is an effective way to identify areas of patient-proxy concordance and discordance about specific treatment preferences. A structured patient-proxy discussion using the LAD helped reconcile discordance and most often in favor of a patient's original wishes.