Abstract Background Bronchiolitis obliterans syndrome (BOS) is a late complication after allogeneic hematopoietic cell transplantation (HCT). While retrospective evidence shows symptomatic respiratory viral infections (RVI) may contribute to the development and progression of BOS, a similar role for asymptomatic RVI is unknown. This study aims to assess the incidence of asymptomatic and symptomatic RVI in a high-risk population. Methods Allogeneic HCT recipients with new-onset chronic graft-versus-host disease (cohort 1, at risk for BOS) and new onset BOS (cohort 2) were enrolled in an ongoing multi-center prospective longitudinal study (NCT05250037) to assess the role of RVI in development of BOS. Participants conducted weekly handheld home spirometry, weekly symptom surveys, and submitted self-administered nasal swabs for respiratory viral PCR testing every two weeks; additional swabs were prompted if symptom survey score reached ≥2. An asymptomatic episode was defined as one or more consecutive positive viral swab with a symptom score of 0-1 and no positive symptoms in the 2 weeks before or after. A symptomatic RVI episode was defined as the first positive PCR to the final positive PCR with any associated symptom score ≥2, allowing no more than ≥4 weeks or 2 negative samples between any 2 consecutive positive PCR samples. Results From March 2022 to July 2023, 73 participants were enrolled (cohort 1: n=50; cohort 2: n=23), with 47 completing one year of follow-up. Compliance was 68% for surveys and 62% for nasal swabs. Twenty percent of 744 swabs tested positive for ≥1 virus. Of 43 participants who tested positive for RVI, 21% (9/43) had 1 episode, 21% (9/43) had 2 episodes, and 52% (25/43) had ≥3 episodes. Ninety-five percent of RVI episodes (104/109) were symptomatic, most commonly rhinovirus (22%), followed by SARS-CoV-2 (21%), seasonal coronavirus (18%), parainfluenza (14%), RSV (6%), adenovirus (5%), and influenza (4%). Only 5% (5/109) of RVI episodes were asymptomatic with 2 episodes of SARS-CoV-2, 2 episodes of seasonal coronavirus, and 1 episode of parainfluenza. The median duration of a symptomatic episode was 1 week (range 1-18 weeks). During the follow-up period, 3 participants in cohort 1 satisfied diagnostic criteria for BOS, of which 2 had antecedent RVI. Conclusion The first year of this observational study revealed frequent symptomatic RVIs and a low rate of asymptomatic RVIs in patients at high risk for BOS. Ongoing follow-up and analyses will evaluate the impact of common RVI on BOS development and progression after HCT.