We evaluated pulmonary vascular permeability with positron emission tomography (PET) in 16 patients with interstitial lung disease (ILD) by measuring the pulmonary transcapillary escape rate (PTCER) for transferrin labeled with gallium-68. In patients with active ILD, defined by lung biopsy or clinical criteria, mean PTCER was significantly greater than in normal subjects (118 +/- 46 versus 21 +/- 11 x 10(-4) min-1, respectively, p less than 0.05). Mean PTCER in patients with inactive ILD, in contrast, was not different from that in normal subjects (32 +/- 10 x 10(-4) min-1, p = NS). Thus, these data suggest that PET measurements of PTCER might serve as an index of disease activity in patients with ILD.