We report the first case of acute renal failure with hyperkalemia associated with the recently marketed direct renin inhibitor aliskiren. To optimize blood pressure control, the antihypertensive medication of a 76-year-old hypertensive female patient was changed from the angiotensin II receptor antagonist irbesartan to aliskiren. Spironolactone was continued, as serum creatinine and potassium levels were initially normal. Two weeks later the patient presented with acute oliguric renal failure, symptomatic hyperkalemia and metabolic acidosis, necessitating emergency dialytic treatment. Unrecognized pre-existing renal insufficiency (CKD Stage 2 - 3) and the continuation of spironolactone were identified as predisposing risk factors.