Although recent X-ray computed tomography (CT) data show that ventral-dorsal gradients in density are common in the adult respiratory distress syndrome (ARDS), we hypothesized that the ventral-dorsal gradient of pulmonary vascular permeability would be evenly distributed in patients with ARDS. We tested this hypothesis by analyzing previously reported data (Am Rev Respir Dis 1991; 143:150-154) obtained by the nuclear medicine imaging technique of positron emission tomography (PET). Measurements of regional lung density (LD), extravascular density (EVD), and the pulmonary transcapillary escape rate (PTCER; an index of vascular permeability) were obtained in eight patients with ARDS and in 10 normal subjects. The data from four contiguous tomographic slices were organized into 20 anatomically oriented "bins" from the ventral (Bin #1) to dorsal (Bin #20) position. Mean overall LD, ERD, and PTCER were all significantly higher (p < 0.05) in the ARDS patients than in normal subjects. Regression slopes of LD versus bin and EVD versus bin were significantly positive (ventral-to-dorsal) in seven of eight and six of eight ARDS patients, respectively. Mean PTCER values showed no difference between ventral and dorsal bin values. A significantly negative slope of PTCER versus bin was detected in five cases and a positive slope in 3, so that the group value was not significant. These data confirm that ventral-dorsal gradients in LD and EVD exist in patients with ARDS. In contrast, no consistent ventral-dorsal distribution of increased pulmonary vascular permeability is present.