A stent graft (1) has at least one fenestration (13) or low profile side arm (34). A guide assembly (17) surrounds the fenestration or low profile side arm. The guide assembly has a continuous wall extending laterally away from the outer surface of the stent graft. The continuous wall is substantially elliptical or circular and extends distally of the fenestration at a distal end of the wall and being coincident with a proximal part (25) of the periphery of the fenestration or low profile side arm at a proximal end of the wall (23). The wall acts to define a guide area to guide a flexible probe extended from a side branch vessel to enter the fenestration or low profile side arm. Where there are two fenestrations or low profile side arms there can be a single continuous wall (50) around both or separate walls for each. The wall may have a peripheral wire reinforcement (19).
