"Gyromitra" are considered to be edible mushrooms although their potential toxicity has been long known. They have caused numerous accidents, sometimes lethal. Historical accounts of poisoning are reported and the authors describe the main characteristics : inconstant toxicity, influence of repetitive ingestions and variable individual sensitivity. Knowing the "gyromitrin" (N-methyl-N-formyl-acetyl-hydrazone) can be converted into methyl-hydrazine, the authors suggest a relation between individual sensitivity to the mushrooms and variation of every body's ability to carry out such a conversion. Several metabolites of gyromitrin can produce enzyme activation ith subsequent synthesis of methylhydrazine. The cumulative activating role of consecutive ingestions is emphasized.