The objective of this study was to (1) determine the incidence of diaphragmatic injuries in penetrating left thoracoabdominal trauma and (2) evaluate the role of laparoscopy in detecting clinically occult diaphragmatic injuries. One hundred nineteen consecutive patients with penetrating injuries to the left thoracoabdominal region presenting to Los Angeles County-University of Southern California Medical Center were prospectively evaluated during an 8-month period. Either celiotomy (with hemodynamic instability or peritonitis) or laparoscopy was performed. Results of the clinical examination and roentgenographic findings were recorded preoperatively. One hundred seven patients were fully evaluated. Fifty patients required emergent celiotomy. Fifty-seven patients underwent laparoscopy. The overall incidence of diaphragmatic injuries was 42% (59% for gunshot wounds, 32% for stab wounds). Among the 45 patients with diaphragmatic injuries, 31% had no abdominal tenderness, 40% had a normal chest roentgenogram, and 49% had an associated hemopneumothorax. Fifteen of the patients undergoing laparoscopy (26%) had occult diaphragm injuries. (1) The incidence of diaphragmatic injuries in association with penetrating left thoracoabdominal trauma is high. (2) The clinical and roentgenographic findings are unreliable at detecting occult diaphragmatic injuries. (3) Laparoscopy is a vital tool for detecting occult diaphragmatic injuries among patients who have no other indications for formal celiotomy.