Options for a hemodynamically stable patient with a penetrating wound to the flank or back but no peritonitis, includes serial physical examinations versus a triple-contrast CT scan. There is, however, little consensus on the minimum time for serial examinations to exclude an injury that requires an operation. Therefore, a retrospective review of patients who sustained a penetrating wound to the flank or back and were admitted to a Level I trauma center was performed. Patients were identified through the trauma registry, patient charts, and morbidity/mortality records. From 1995 to 2003, 93 patients undergoing observation for a penetrating flank/back wound subsequently required a therapeutic laparotomy. The time from admission to operation was less than 3 hours for 84 per cent of the patients requiring therapeutic intervention. A further 10 per cent presented with symptoms between 4 to 6 hours, and 6 per cent between 7 to 18 hours. All the injuries caused symptoms within 18 hours of the injury event. The majority of patients (94%) who require a laparotomy after a period of observation for a penetrating flank/back wound will develop signs and symptoms within 6 hours of admission. A period of hospitalization longer than 18 hours did not detect further injuries in the asymptomatic patient.