The authors investigated prospectively whether diagnostic peritoneal lavage (DPL) would aid in diagnosing of patients with suspected peritonitis unrelated to trauma. Fifty patients whose diagnosis were obscure by conventional methods were included in this study. Lavage fluid was examined for gross apperance and analyzed for white blood cell (WBC) and red blood cell (RBC) counts, amylase and bilirubin levels. Elevation of WBC count in the lavage fluid was found to be the most important criterion indicating acute peritonitis, whereas elevated lavage amylase level alone was misleading in most cases. The authors concluded that, though DPL was a reliable procedure in detecting whether peritonitis is present or not, standard criteria for positive peritoneal lavage had to be revised in acute abdominal diseases.