Gyromitra esculenta is an ascomycete fungus from the genus Gyromitra. The fruiting body, or mushroom, is an irregular brain-shaped cap, dark brown in colour, that can reach 10 centimetres (4 inches) high and 15 cm (6 in) wide, perched on a stout white stipe up to 6 cm (2+1⁄2 in) high. It is widely distributed across Europe and North America, normally fruiting in sandy soils under coniferous trees in spring and early summer.

Although potentially fatal if eaten raw, G. esculenta is sometimes parboiled for consumption, being a popular delicacy in Europe and the upper Great Lakes region of North America. However, evidence suggests that thorough cooking does not eliminate all toxins. When consumed, the principal active mycotoxin, gyromitrin, is hydrolyzed into the toxic compound monomethylhydrazine, which affects the liver, central nervous system, and sometimes the kidneys. Symptoms involve vomiting and diarrhea several hours after consumption, followed by dizziness, lethargy and headache. Severe cases may lead to delirium, coma, and death. Recent evidence suggests G. esculenta may be chronically toxic and linked to regional clusters of ALS.

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