To evaluate the patterns of normal lung tissue, alveolar and interstitial oedema, macrophage collections, haemorrhage and infarction in radiographs of excised air-inflated lungs at autopsy, the distribution of these findings in 89 lungs was determined histologically on 441 sections sampled from 59 lungs selected to represent unequivocally the features in question. Normal lung tissue appeared as branching blood vessels with a delicate net-like background. Alveolar oedema gave a fine punctate opacity distributed evenly, and interstitial oedema showed as either short lines perpendicular to the pleural surface, as a network of branching narrow lines, or as long non-branching lines. Macrophage collections associated with chronic congestion showed as a coarsely punctate opacity, but this was often inconspicuous or similar to the pattern of slight alveolar oedema or slight haemorrhage. Massive haemorrhage appeared as a finely macular opacity. Fresh infarction gave a dense wedge-shaped opacity with pleural retraction and a rounded top. An older infarction was less homogeneous with more indistinct borders. The demonstration of thromboemboli without infarction was not possible.