Single-lung transplantation has become a treatment option for many patients with advanced pulmonary disease. Recent advances in surgical technique and refined immunosuppressive regimens have led to improvement in long-term outcomes, but postoperative complications, including airway disorders, remain problematic. Serial spirometry with flow-volume loops is sensitive in detecting early small-airway disease associated with lung allograft rejection or bronchiolitis obliterans, but its role in the diagnosis of large-airway disease in the posttransplantation setting has not been delineated. In this report, we describe a novel alteration in the configuration of the flow-volume loop in a patient who developed unilateral mainstem bronchial obstruction following single-lung transplantation for severe emphysema. Surveillance spirometry performed 6 weeks after transplantation demonstrated a new initial plateau in the maximal expiratory flow-volume curve suggestive of a variable intrathoracic airway obstruction. This unique aberration in the flow-volume curve sheds new insight into the physiologic abnormalities of spirometry in patients receiving lung transplants.