HE FUNCTIONAL AND STRUCTURAL STATUS OF THE pulmonary vascular bed plays a pivotal role in the presentation and outcome of the child with congenital cardiovascular disease. However, it is in the immediate postoperative period that the child is most vulnerable to a sudden or sustained increase in pulmonary vascular resistance. Following surgery for congenital cardiac disease pulmonary vascular reactivity is heightened and vasospastic stimuli may result in sudden increases in pulmonary arterial pressure and resistance resulting in acute right heart failure, tricuspid regurgitation, systemic hypotension, myocardial ischemia and increased airway resistance. These episodes, called pulmonary hypertensive crises, may be lethal events. Furthermore, mildly stimulating events precipitate similar crises, and the crises tend to last longer and cluster. 1,2 Clearly, the pathophysiology of such events is complex and incompletely understood by the analysis or measurement of a single vasoactive mediator. Postoperative pulmonary hypertension represents a complex interplay between the preoperative condition of the patient (importantly age at repair, type of lesion and presence of a syndrome) and the inevitable disruption in the endocrine and vasoactive peptide milieu that results from cardiac surgery. Important contributors to a milieu of enhanced vasoconstriction are cardiopulmonary bypass, hypothermia and circulatory arrest. Residual cardiac lesions and the sequelae of the stress response, hypoxia, metabolic and respiratory acidosis may all contribute additional imbalances favoring pulmonary vasoconstriction. Postoperative sequelae such as right and left ventricular and atrioventricular valve dysfunction will be important in determining how well the postoperative elevation in pulmonary vascular resistance is tolerated. Currently, endothelial cell dysfunction, present preoperatively and exacerbated by perioperative influences is considered a unifying hypothesis to account for many