Mushrooms of the ubiquitous Cortinarius species (Cs) contain nephrotoxins that can cause acute and chronic renal failure by an unknown pathomechanism. Typical is a long symptom-free interval before the onset of clinical disease. A causal form of therapy is not known. Early hemodialysis can improve the prognosis of this potentially life-threatening condition. Diagnosis of Cs poisoning can be made by detecting the responsible toxin--orellanine--in plasma or renal tissue by fluorimetry after thin-layer chromatography or by identifying the spores of left-over mushrooms as Cs. Renal histology shows nonspecific changes such as tubular dilatation and flattening of the epithelium and signs of interstitial edema followed by interstitial fibrosis. We present four cases of Cs poisoning with different outcomes and a review of the literature.