A mediastinal tumour was simulated by gross dilatation of the azygos vein resulting from compression by enlarged hilar lymph nodes at the point where it crossed the right main bronchus to enter the superior vena cava. This cause of dilatation of the azygos vein has not previously been reported. Published reports are reviewed and attention drawn to the large proportion of patients (11/19) with this condition who had needless thoracotomies. The possibility of making a diagnosis by means of clinical observation and simple radiological techniques and thus avoiding surgical intervention is discussed.