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.