Despite advances in the management of myeloablative allogeneic hematopoietic stem cell transplants, airflow obstruction (AFO) remains a significant complication. We conducted a 12-year study to examine the recent epidemiology of AFO and its associated mortality. Using the rate of percent predicted FEV1 decline after transplant, we defined AFO as a more than 5% per year decline in percent predicted FEV1 with the lowest post-transplant FEV1/FVC ratio less than 0.8. New obstruction was more frequent than previous estimates (26% overall, 32% among patients with chronic graft-versus-host disease [GVHD]) and was significantly associated with older age at transplant, lower pretransplant FEV1/FVC ratio, history of both acute and chronic GVHD, and respiratory viral infection within the first 100 days after transplant. AFO was associated with significant attributable mortality rates of 9% at 3 years, 12% at 5 years, and 18% at 10 years after transplant, which were much higher for the subpopulation of patients with chronic GVHD (22% at 3 years, 27% at 5 years, and 40% at 10 years). These results suggest that the incidence of AFO may have been underestimated previously, and its presence significantly increases the mortality of long-term survivors of myeloablative allogeneic hematopoietic stem cell transplant patients.