Four cases of Mendelson's syndrome (acid pulmonary aspiration) are presented. They all demonstrate an acute diffuse alveolar filling pattern. This appearance is by no means specific. However, in the absence of other causes for this pattern and of evidence of left ventricular failure the radiologist may alert the clinician to the correct diagnosis. Early recognition of this syndrome will result in prompt treatment which differs significantly from that of other causes of this radiographic appearance. The differential diagnosis is discussed.