Ninety-seven consecutive patients admitted to the ICU with peritonitis were studied. Of these, 66 (68%) died and autopsies were performed on 38 (58%). Twelve of the 38 autopsied patients showed pathologic criteria of adult respiratory distress syndrome (ARDS); 26 did not. Chest x-rays of the 38 autopsied patients were examined and 9 (75%) of the 12 patients with histologic criteria of ARDS, specifically, moderate or severe fibrosis, had decreased pulmonary longitudinal diameter on serial chest x-rays. The remaining 3 (25%) patients did not have this specific finding. Of the 26 patients without pathologic criteria of ARDS, only 2 (8%) had reduction in pulmonary longitudinal diameter. We conclude that a marked reduction in the pulmonary longitudinal diameter is highly suggestive of ARDS and especially useful in differentiating ARDS from bilateral pneumonia.