In 1991, the Nordic Working Group on Food Toxicology and Risk Evaluation (NNT) reviewed the available data on phenylhydrazines naturally occurring in the cultivated mushroom. It was concluded that the mushroom may contain about 500 mg of the hydrazine derivatives per kg fresh weight. The hydrazine derivatives as well as extracts of the cultivated mushroom were mutagenic to a variable degree in most of the reported short-term tests. The raw mushroom and several of the hydrazines induced tumours when administered to Swiss mice as reported by American scientists. However, reservations were expressed as to the design of the studies. Based on this review, and due to the concern expressed, a Nordic project (coordinated by Jørn Gry, Danish Veterinary and Food Administration) was initiated dealing with toxicological and chemical studies on the cultivated mushroom and its phenylhydrazine derivatives in order to provide data for the evaluation of whether the consumption of the cultivated mushroom constitutes a human risk. The present report summarises the Nordic seminar which had the aim to present Nordic studies and to promote exchange of information between chemists and toxicologists in the field of Agaricus bisporus research. American, English and Czech researchers gave an up-to-date overview on the cultivated mushroom and its hydrazines and reviewed their ongoing research. Finally, Nordic researchers summarised their chemical and toxicological studies on behalf of the Nordic Project Group on Phenylhydrazines in the Cultivated Mushroom". This is the 7th Nordic Report from the project group on inherent natural toxicants in food.