OBJECTIVES: Taxonomy of injuries involving the costal margin is poorly described and surgical management varies. These injuries, though commonly caused by trauma, may also occur spontaneously, in association with coughing or sneezing, and can be severe. Our goal was to describe our experience using sequential segmental analysis of computed tomographic (CT) scans to perform accurate assessment of injuries around the costal margin. We propose a unifying classification for transdiaphragmatic intercostal hernia and other injuries involving the costal margin. We identify the essential components and favoured techniques of surgical repair. METHODS: Patients presenting with injuries to the diaphragm or to the costal margin or with chest wall herniation were included in the study. We performed sequential segmental analysis of CT scans, assessing individual injury patterns to the costal margin, diaphragm and intercostal muscles, to create 7 distinct logical categories of injuries. Management was tailored to each category, adapted to the individual case when required. Patients with simple traumatic diaphragmatic rupture were considered separately, to allow an estimation of the relative incidence of injuries to the costal margin compared to those of the diaphragm alone. RESULTS: We identified 38 patients. Of these, 19 had injuries involving the costal margin and/or intercostal muscles (group 1). Sixteen patients in group 1 underwent surgery, 2 of whom had undergone prior surgery, with 4 requiring a novel double-layer mesh technique. Nineteen patients (group 2) with diaphragmatic rupture alone had a standard repair. CONCLUSIONS: Sequential analysis of CT scans of the costal margin, diaphragm and intercostal muscles defines accurately the categories of injury. We propose a 'Sheffield classification' in order to guide the clinical team to the most appropriate surgical repair. A variety of surgical techniques may be required, including a single- or double-layer mesh reinforcement and plate and screw fixation.