We describe an unusual case of a large Aorto-pulmonary window which presented as Eisenmenger's syndrome at the age of 20 years in a male patient. We take this opportunity to highlight some of the basic investigations like Chest X Ray and Contrast Echocardiography which can delineate the level of the shunt. Attention should be given to the subtle bedside clinical signs which can give clues to the diagnosis. We did a cardiac CT scan for confirmation of the diagnosis.