During an anatomic dissection of the posterior mediastinum, an unusual course of the interazygos v. was noted. The hemiazygos v. ascended to the 8th thoracic vertebra, in front of the vertebral column. It left the column, ascending obliquely, and passing in front of the aorta and behind the oesophagus. This transverse segment, crossing the midline, is called the interazygos v. It may be confused with different lesions situated at this level: aneurysm, tumour or lymph node hypertrophy, during CT or MRI studies. Secondly, the surgeon must be aware of this azygos variation as a possible cause of haemorrhage during a procedure in this region.