We evaluated pulmonary vascular permeability in 15 patients after lung transplantation (21 allografts) by measuring the pulmonary transcapillary escape rate (PTCER) for Ga-68-labeled transferrin, using positron emission tomography. Seven recipients (four unilateral, three bilateral lung transplants) were studied within 3 days of transplantation, and each developed hypoxemia and allograft infiltrates consistent with the "reimplantation response." PTCER was higher in subjects studied within 1 day than in those studied at a later time, and fell in seven allografts studied serially. The initial PTCER also correlated (r = 0.77) with length of ischemic (preservation) time, even in the three subjects with bilateral allografts. Eight other recipients (five unilateral, three bilateral transplants) were evaluated for possible organ rejection at least 1 wk after transplantation. PTCER was normal in patients without clinical or histologic evidence of rejection, and it was elevated in recipients with rejection. PTCER fell each time after treatment for rejection with increased immunosuppression in the three patients studied serially. These data suggest that positron emission tomography measurements of PTCER might be a useful way to evaluate both the reimplantation response and organ rejection after lung transplantation.