Fifty-one patients with penetrating wounds of the thorax at or below the 4th intercostal space were evaluated with peritoneal aspiration and lavage. Nineteen gunshot wounds and 32 stab wounds were studied and there was a 22% incidence of diaphragmatic penetration and/or intra-abdominal injury. There were ten true positive and one false positive study. There were 39 true negative and one false negative lavage. Peritoneal aspiration and lavage is a test of high sensitivity (91%) and specificity (98%). It has proven to be a useful screening technique in patients with penetrating wounds of the thorax. Peritoneal aspiration and lavage identify the patients with diaphragmatic penetration and intra-abdominal injury and minimize negative exploration.