The renin-angiotensin-aldosterone system appears to play an important initiating role in the pathogenesis of hypertension in patients with autosomal-dominant polycystic kidney disease (ADPKD). Therefore, angiotensin-converting enzyme (ACE) inhibitors would appear to be appropriate therapy for hypertension in such patients. However, because ADPKD is a bilateral disorder, ACE-inhibitor therapy may worsen renal function, as occurs in patients with bilateral renal artery stenosis. We describe eight episodes of reversible acute renal deterioration in five patients with ADPKD, massive renal involvement, and chronic renal insufficiency. In all cases, ACE-inhibitor therapy either predisposed the patient to or precipitated the acute event. Two of the patients who had acute renal failure while receiving ACE-inhibitor therapy experienced a recurrence when rechallenged with such therapy. Furthermore, patients receiving combined therapy with an ACE inhibitor and a diuretic and patients who experience a cyst hemorrhage while receiving ACE inhibitor therapy are also at risk for reversible renal dysfunction. Caution is therefore recommended in using ACE inhibitors to treat hypertension in patients with ADPKD who are at high risk because of compromised renal function and massive cystic involvement.