This report describes a case of multiple anomalies of the azygos venous system. On the left side, the accessory hemiazygos vein was formed by the 4th - 6th intercostal veins ventral to the aorta. The 7th and 8th intercostal veins united to form a common stem that joined the accessory hemiazygos vein at the level of the 8th thoracic vertebra to create a common trunk. The hemiazygos vein formed by the 9th - 11th intercostal veins joined the common trunk to form a dilated vein termed the interazygos vein. This vein crossed the descending aorta ventrally to open into the azygos vein on the right side. Anomalies of the azygos venous system are common, but the incidence of hemiazygos and accessory hemiazygos veins communicating ventral to the aorta is very low. The development of communications between the left intersegmental veins and the azygos vein ventral to the descending aorta could explain such anomalies. Abnormal communications of these veins anterior to the aorta may mimic aneurysms, tumors and enlarged lymph nodes, and could be misinterpreted by radiologists in computed tomography and magnetic resonance imaging scans. Surgeons operating on the posterior mediastinum must be aware of such anomalies in order to avoid accidental damage to the blood vessels in this region.