Two men (61 and 81 years old) with mild impaired kidney function developed acute renal failure due to dehydration combined with the use of inhibitors of the renin-angiotensin-aldosterone system (RAAS). After rehydration, correction of hyperkalaemia and stopping RAAS-inhibition and diuretics, they recovered completely. Many patients using RAAS-inhibitors have impaired renal function. In the case of dehydration due to gastroenteritis or prolonged fever they risk developing acute renal failure. The high risk groups are elderly patients, patients with atherosclerosis or heart failure and those with co-medication of diuretics or NSAIDs. The underlying mechanism is that the normal pathways to protect kidney perfusion in case of hypovolaemia are blocked by the use of RAAS-inhibitors or NSAIDs. In the case of dehydration in patients with chronic kidney disease using RAAS-inhibitors, serum creatinine and potassium levels should be monitored. Temporary discontinuation of RAAS-inhibitors or diuretics is often necessary.