BACKGROUND/PURPOSE: Accessory hemi diaphragm (AHD) is an uncommon condition. Its clinical features, diagnosis and management have not been clearly defined. METHODS: We reviewed the world literature and added a case of our own. RESULTS: There are 40 proven cases in the literature. The lesion occurs almost exclusively in the right hemithorax. Twenty seven cases presented with cardiovascular and/or respiratory symptoms while 3 had symptoms pertaining to other systems. In the remaining 10 patients the diagnosis was either made accidentally or information pertaining to their presentation is missing. Abnormalities of pulmonary and systemic blood vessels often accompany the abnormality. The diagnosis is usually suspected because of an abnormal chest radiograph and confirmed by a variety of tests. CONCLUSIONS: If the AHD prevents proper aeration of the lung it should be excised. If vascular abnormalities coexist they should be treated along with the AHD. Asymptomatic patients may be observed after confirming the diagnosis.