Mushrooms of the Cortinarius species are nephrotoxic and can cause severe acute renal failure. The toxic effect is due to orellanine. It is suspected that the cytotoxic damage is caused by the production of oxygen-free radicals. Renal pathology shows tubular necrosis with interstitial nephritis. In addition to accidental intoxications as a consequence of mushroom meals, recent cases are often due to voluntary abuse of natural drugs like magic mushrooms. We report 4 current cases of acute renal failure from intoxication by Cortinarius species by confusing it with psychoactive fungi. Typical for the Cortinarius poisoning is the long latency period from ingestion until the onset of clinical symptoms (3 - 20 days). Diagnosis is based on microscopical identification of the mushroom spores, and detection of the orellanine toxin in leftover mushrooms. In renal biopsy tissue, orellanine is detectable by thin-layer chromaography technique up to 6 months after poisoning. There is no causative therapy, and treatment is symptomatic with adequate hemodialysis. In cases of otherwise unexplained acute renal failure, intoxication with nephrotoxic mushrooms should be considered.