e23064 Background: Caregivers of allogeneic hematopoietic stem cell transplant (allo-HSCT) recipients bear significant responsibilities involving commitment to be available 24/7 up to at least 100 days post-transplant to provide emotional and physical support to their patients. Caregiving for such an extended duration is associated with reduced quality of life, compromised psychological well-being, and dysregulated metabolic and immunological functioning. Ecological momentary assessment (EMA) may allow a better understanding of the dynamic day-to-day experiences of this unique population. However, no prior research on EMA exists in this population. This study reports on the feasibility of delivering EMAs in the form of a daily diary (DD) and how completion of the DD relates to the patient’s course of treatment and caregiver’s demographics among the caregivers of allo-HSCT caregivers. Results are part of an ongoing 3-arm randomized controlled trial (71% enrolled to date) of those individuals randomized to the usual care condition. Methods: Caregivers (n = 46) received DD prompts for 6 weeks during their patient’s active treatment phase: before transplant (week 1), transplant through discharge (weeks 2 and 3), after discharge (weeks 3 through 6). The DD included six affect items from the Positive and Negative Affect Schedule (stressed, overwhelmed, frustrated, drained, guilty, and hopeful), three state mindfulness items (attention, non-judgement, and decentering), and one item related to caregiver’s self-efficacy in patient care. Results: Among 46 caregivers (mean age = 55.8 [SD = 15.7], 74% female, 32% retired), the overall completion rate (CR = total number of prompts completed/total number of prompts received) was 65.5%, with 72% of participants completing > 50% and 22% of participants completing > 85% of the prompts. For the duration before transplant, the overall CR was 63.2%, with 66.7% of participants completing > 50%, and 33.3% completing > 85% of the prompts. For the duration between transplant through discharge, the overall CR was 65.57%, with 76% of participants completing > 50%, and 26% completing > 85% of the prompts. Similarly, for the duration after discharge, the overall CR was 65.14%, with 66.7% completing > 50%, and 28.9% completing > 85% of the prompts. DD completion was not related to the caregiver’s age, sex, or employment status ( ps > 0.1). Conclusions: The completion rate for the DD was very similar across the patient’s course of treatment, suggesting EMA completion is not excessively burdensome for caregivers potentially experiencing dynamic distress. Furthermore, EMA completion was not associated with age, sex, or employment status, indicating that all caregivers, regardless of demographics, were amenable to completing EMAs. Our findings suggest that delivering EMAs is feasible in this population. Clinical trial information: NCT05078229 .