Forty-five consecutive patients with penetrating thoraco-abdominal trauma underwent surgical exploration to evaluate the ability of peritoneal lavage to detect peritoneal penetration. Eight patients fulfilled standard criteria for operation and did not undergo lavage. The remaining 37 patients underwent diagnostic peritoneal lavage using a closed technique before exploratory laparotomy. Using 10,000 RBC/mm3as our previously established criterion for peritoneal penetration, there were seven true positive, one false positive, 28 true negative, and one false negative lavage for an overall accuracy of 94.6% with 87.5% sensitivity and 96.6% specificity as determined by subsequent laparotomy. While 33% of this patient cohort were found to have significant injuries (four had isolated diaphragmatic injuries, all detected by peritoneal lavage), 67% were subjected to negative surgical exploration, as accurately predicted by peritoneal lavage. Negative laparotomy carried a 10.7% operative morbidity. Based on these data we advocate diagnostic peritoneal lavage in patients with thoraco-abdominal penetrating trauma who otherwise lack operative indications.