Use of stem cell transplant (mostly for hematologic cancers) is expected to increase 5-fold by 2030, when the number of survivors will reach 500,000. This highly toxic treatment causes a range of acute physical and psychological symptoms (e.g., pain, fatigue, distress) which then persist for years for up to 45% of patients. To address gaps in existing symptom-focused behavioral interventions that can be added to standard patient care, we developed Expressive Helping (EH), a low-cost, low-burden intervention that targets a range of physical and psychological symptoms in a broad group of transplant recipients. EH includes 2 therapeutic components, completed in 4 brief structured writing sessions over 4 weeks: 3 sessions of Pennebaker’s emotionally expressive writing followed by 1 session of theory- and evidence-based “peer support” writing in which patients use insights from expressive writing to write an account of their transplant experience that is meant to be shared to help people preparing for or undergoing transplant. In a randomized controlled trial (RCT) comparing EH to 3 active control arms in long-term transplant survivors, EH reduced physical symptoms and distress in survivors with moderate to severe persistent symptoms. Because symptoms are elevated for most patients as they undergo transplant, and early symptoms predict persistent symptoms, we now propose to determine whether EH can be used during transplant to reduce common acute symptoms and prevent development of persistent symptoms. Further, EH was designed to be part of a complementary, 2-part “reciprocal benefits of helping” intervention strategy based on evidence that patients can benefit both from getting experiential information about transplant from other patients’ peer support narratives and from providing peer support by completing EH. We therefore propose to begin development and evaluation of a web-based supportive resource to deliver peer support narratives from EH to patients preparing for and undergoing transplant. Thus, in Aim 1 we propose to conduct a 2-arm RCT to evaluate the efficacy of EH when used in the early posttransplant period. Participants will be 315 cancer patients with at least moderate symptoms, recruited pretransplant and randomized to EH or a neutral writing (NW) time and attention control arm. They will complete their assigned writing from the first week of hospitalization to 2 weeks post-discharge and be followed for 12months to evaluate effects on symptom burden (primary outcome) and psychological distress, quality of life, and performance status (secondary outcomes). In Aim 2 we will evaluate potential intervention mechanisms (theorized causal pathways) and moderators. In Aim 3 we will develop and evaluate optimal methods for delivering peer support narratives from EH to patients preparing for or undergoing transplant, guided by data from trial participants, a community advisory board of key stakeholders, focus groups, and usability testing. This proposed integrated approach is novel and holds promise as a disseminable and clinically-relevant intervention strategy for reducing acute and persistent symptoms in stem cell transplant recipients.