The study was aimed at determining the value of peritoneal lavage as an aid in the diagnosis of acute peritonitis when the diagnosis could not be made by ordinary clinical criteria. Thirty patients were studied prospectively. Twenty-five of the patients were suspected of having peritonitis but doubt existed because of an unreliable history or equivocal or confusing abdominal signs. Five remaining patients were comatose and an intraabdominal source of sepsis needed to be excluded. The technique of lavage and criteria for diagnosis have been presented herein. There were 10 true-positive results, no false-positive results, 19 true-negative results, and one false-negative result (positive predictive value 100 percent, confidence level 95 percent). There were no complications due to the test. Peritoneal lavage is a useful method of diagnosing or excluding peritonitis in patients with confusing abdominal signs. Negative results of laparotomy or prolonged observation may thus be obviated in such patients.