Mycetismus or mushroom poisoning is defined in Anisworth and Brisby's Dictionary of the Fungi as distress resulting from the consumption of a fungal organism. This is in contrast to mycotoxicosis where the fungus, usually a hyphomycete, acts on some foodstuff of humans or animals and alters it. The paper describes three incidents of poisoning by the mushroom Amanita virosa in the Hamadan region of Iran. Two poisoning incidents occurred in adults and one in a child. All patients were associated with severe gastrointestinal symptoms including profuse diarrhea, vomiting and intestinal pain. For identification of A. virosa, in addition to the case history and the symptoms, examination of kitchen waste was done and mushroom remains examined with 2% aqueous KOH, Melzer's reagent, using the identification keys of Bresinsky. Because this fungus is likely to be confused with edible mushrooms and mushroom hunting is a popular endeavor in some areas of the Hamadan region, it is important to educate people about the dangers involved.