A stent graft (2) for placement in the thoracic arch of a patient has a first tubular body portion (6) with a first lumen therein for placement in the ascending aorta of a patient and a second tubular body portion (8) to extend along the thoracic arch and down the descending aorta. The second tubular body portion is of a lesser diameter than the first tubular body portion. There is a step portion (10) between the first body portion and the second body portion. The step portion is joined to and continuous with the first portion and the second portion. A first side of each of the first body portion, the step portion and the second body portion are substantially aligned so that there is a step (18) defined on a second side opposite to the first side of the body portion. There is an aperture (30) in the step portion and an internal tube (32) extending from the aperture towards the first body portion. The internal tube is divided along part of its length into at least two smaller internal tubes (34, 36) with the smaller internal tubes opening into the first lumen.