Single kidney cysts are one of the commonest kidney lesions although they are seldom related to high blood pressure. Contrariwise, polycystic kidney disease, either autosomal dominant or recessive, is frequently associated with arterial hypertension. The mechanisms involved in the pathogenesis of hypertension and the decline of renal function in polycystic kidney disease are not yet completely understood, but it seems that uncontrolled blood pressure is involved in the progression of kidney failure. Therefore, measurement of blood pressure (including ambulatory blood pressure monitoring) in these patients, even when they are still asymptomatic children, is needed. Quick treatment of high blood pressure should be performed and renin-angiotensin-axis blockers are usually recommended to start the antihypertensive therapy. All these issues are reviewed in this chapter.