Abstract Background: To evaluate the incidence of occult diaphragmatic injuries and investigate the role of laparoscopy in patients with penetrating trauma to the left lower chest who lack indications for exploratory celiotomy other than the potential for a diaphragm injury. Study Design: Patients with penetrating injuries to the left lower chest who were hemodynamically stable and without indications for a celiotomy were prospectively evaluated with diagnostic laparoscopy to determine the presence of an injury to the left hemidiaphragm. Diagnostic laparoscopy was performed in the operating room under general anesthesia. Results: One-hundred-ten patients (94 stab wounds, 16 gunshot wounds) were evaluated with laparoscopy. Twenty-six (24%) diaphragmatic injuries were identified (26% for stab wounds and 13% for gunshot wounds). Comparison of patients with diaphragmatic injuries with those without diaphragmatic injuries demonstrated a slightly greater incidence of hemo/pneumothoraces (35% versus 24%, NS). The incidence of diaphragmatic injuries in patients with a normal chest x-ray was 21% versus 31% for patients with a hemo/pneumothorax. An elevated left hemidiaphragm was associated with a diaphragmatic injuries in only 1 of 7 patients (14%). The incidence of diaphragmatic injuries was similar for anterior, lateral, and posterior injuries (22%, 27%, and 22% respectively). Conclusions: The incidence of occult diaphragmatic injuries in penetrating trauma to the left lower chest is high, 24%. These injuries are associated with a lack of clinical and radiographic findings, and would have been missed had laparoscopy not been performed. Patients with penetrating trauma to the left lower chest who do not have any other indication for a celiotomy should undergo videoscopic evaluation of the left hemidiaphragm to exclude an occult injury.