Isolated partial or complete absence of the coronary sinus roof hemodynamically behaves like an atrial septal defect and rarely causes pulmonary hypertension. We report a case of unroofed coronary sinus (URCS) presenting with post-operative cyanosis due to pulmonary hypertensive crisis following surgical repair of a ventricular septal defect at 6 months of age. He had progressively decreased stamina with gradual increase in pulmonary pressure requiring surgical closure of the URCS by 6 years of age with resolution of symptoms and normalization of the RV pressure.