Despite the heterogeneous distribution of pulmonary blood flow, perfusion appears to be spatially ordered, with neighboring regions of lung having similar magnitudes of flow. This premise was tested by determining the spatial correlation of regional flow [rho(d)] as a function of distance (d) between regions. Regional pulmonary perfusion was measured in both supine and prone positions in seven anesthetized mechanically ventilated dogs with radiolabeled microspheres. After excision and drying, the lungs were cubed into pieces 1.2 cm on a side, with a three-dimensional coordinate assigned to each piece. The microsphere-determined flow to each piece was measured by radioactive counts, and rho(d) was calculated for all paired pieces within the same lobe. rho(d) was greatest for adjacent pieces (d = 1.2 cm) and decreased with increasing d, becoming negative at large distances in all dogs and positions. The spatial correlation of flow between adjacent pieces, rho(1.2 cm), was greater in the supine than in the prone position (0.66 vs. 0.72, P less than 0.05). The observations for each dog and position were fit to the equation rho(d) = d(a)+b.d+c, and the coefficients were used to compare rho(d) in the supine and prone positions. rho(d) differed in the two positions (P less than 0.05), with rho(d) falling off more rapidly with distance in the supine position. When trends in flow due to gravity were mathematically removed, differences between supine and prone positions were no longer observed. The spatial correlation of regional pulmonary perfusion was anisotropic in both supine and prone positions. The observation that regional pulmonary perfusion is highly correlated over large spatial distances has important implications for models of flow distribution.