Acute renal failure is a known complication during hemolytic crisis in paroxysmal nocturnal hemoglobinuria (PNH). However, chronic renal failure is rare despite the well-known spectacular hemosiderosis of the kidneys due to chronic hemolysis. Here, we report about a 74-year-old man with PNH who developed acute on chronic renal failure after an episode of intercurrent urinary tract infection and subsequent hemolytic crisis. Mild chronic hemolysis, well-documented over the past decade, had long been considered the cause of a constantly declining glomerular filtration rate. Accordingly, magnetic resonance imaging during admission demonstrated marked siderosis of both kidneys, supporting the hypothesis that chronic renal failure (CRF) was likewise related to PNH. However, a renal biopsy revealed acute tubular necrosis and distinct renal siderosis, as expected. Additionally, tubulointerstitial injury and global glomerular sclerosis, best classified as arterionephrosclerosis, were present. In retrospect, these findings were explained by a 15-year history of hypertension and a 4-year medication with cyclosporine. Careful diagnostic workup including a renal biopsy is mandatory, given a misleadingly suggestive correlation between chronic hemolysis and CRF. Chronic renal failure in PNH is a diagnosis of exclusion, even if radiologic evidence of heavy siderosis draws off the physician's attention.