Coarctation of the aorta is the most frequent cause of hypertension in infants and children. Ninety-eight per cent of coarctations occur in the descending aorta near the ligamentum arteriosus. Five patients are presented with the relatively rare problem of coarctation of the abdominal aorta. The anatomic,pathophysiologic and clinical aspects in these patients cover a range of variants. Clinical and laboratory studies of the genesis of hypertension in coarctation are reviewed in chronologic outline. An experimental model of abdominal coarctation with hypertensive and renin-angiotensin II correlations suggests but does not prove a renal mechanism for the hypertension. The same conclusion must be drawn from study of the clinical cases.