To analyze the effectiveness of peritoneal lavage and to define its limitations in the evaluation of patients who have sustained blunt abdominal trauma, a prospective study of 500 such patients was undertaken by the Trauma Service at the Naval Hospital, San Diego. Utilizing a qualitative colorometric method to evaluate the degree of hemoperitoneum, patients could rapidly be divided into three clinical groups: strongly positive, weakly positive, and negative. Using this method, patients with a strongly positive peritoneal lavage had a 94% incidence of significant intra-abdominal injuries. In 333 patients with a negative lavage, there was no documented incidence of significant intra-abdominal injuries. Visceral angiography and abdominal echography were utilized in this group of patients to identify those with significant intra-abdominal injuries. By utilizing this approach, there were only eight unnecessary celiotomies in the total group of 500 patients. It is concluded, therefore, that peritoneal lavage is a safe, rapid, and effective means of evaluating patients who have sustained blunt abdominal trauma.