Paxillus involutus, also known as the brown roll-rim or the common roll-rim, is a species of basidiomycete fungus. Although it has gills, it is more closely related to the pored boletes than to typical gilled mushrooms. It was first described by Pierre Bulliard in 1785, and was given its current binomial name by Elias Magnus Fries in 1838. Genetic testing suggests that Paxillus involutus may be a species complex rather than a single species. Various shades of brown in colour, the fruit body grows up to 6 cm (2+3⁄8 in) high and has a funnel-shaped cap up to 12 cm (4+3⁄4 in) wide with a distinctive inrolled rim and decurrent gills that may be pore-like close to the stipe. The species is widely distributed across the Northern Hemisphere. It has been inadvertently introduced to Australia, New Zealand, South Africa, and South America, probably transported in soil with European trees. It is common in deciduous and coniferous woods and grass in late summer and autumn, forming ectomycorrhizal relationships with a broad range of tree species. These benefit from the symbiosis as the fungus reduces their intake of heavy metals and increases resistance to pathogens such as Fusarium oxysporum. Previously considered edible and eaten in parts of Europe, it has been found to be deadly poisonous, after being responsible for the death of German mycologist Julius Schäffer in 1944. It had been recognized as causing gastric upsets when eaten raw, but was more recently found to cause potentially fatal autoimmune hemolysis, even in those who had consumed the mushroom for years without any other ill effects. An unknown antigen in the mushroom triggers the immune system to attack red blood cells. Serious and commonly fatal complications include acute kidney injury, shock, acute respiratory failure, and disseminated intravascular coagulation.

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