Paxillus involutus, commonly known as the brown roll-rim, common roll-rim, or poison pax, is a basidiomycete fungus widely distributed across the Northern Hemisphere. It has been unintentionally introduced to Australia, New Zealand, and South America, where it has likely been transported in soil with European trees. Various shades of brown in colour, the fruit body grows up to 6 cm (2.4 in) high and has a funnel-shaped cap up to 12 cm (5 in) wide with a distinctive inrolled rim and decurrent gills that may be pore-like close to the stipe. 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.
A common mushroom of deciduous and coniferous woods and grassy areas in late summer and autumn, Paxillus involutus forms 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 widely in Eastern and Central Europe, it has since been found to be dangerously poisonous, responsible for the death of German mycologist Julius Schäffer in 1944. It had been recognised 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 antigen in the mushroom triggers the immune system to attack red blood cells. Serious and potentially fatal complications include acute renal failure, shock, acute respiratory failure, and disseminated intravascular coagulation.
Resembling a brown wooden top, the epigeous (aboveground) fruit body may be up to 6 cm (2.4 in) high. The cap, initially convex then more funnel-shaped (infundibuliform) with a depressed centre and rolled rim (hence the common name), may be reddish-, yellowish- or olive-brown in colour and typically 4–12 cm (1.6–5 in) wide; the cap diameter does not get larger than 15 cm (5.9 in). The cap surface is initially downy and later smooth, becoming sticky when wet. The cap and cap margin initially serve to protect the gills of young fruit bodies: this is termed pilangiocarpic development. The narrow brownish yellow gills are decurrent and forked, and can be peeled easily from the flesh (as is the case with the pores of boletes). Gills further down toward the stipe become more irregular and anastomose, and can even resemble the pores of bolete-type fungi. The fungus darkens when bruised and older specimens may have darkish patches. The juicy yellowish flesh has a mild to faintly sour or sharp odor and taste, and has been described as well-flavored upon cooking. Of similar colour to the cap, the short stipe can be crooked and tapers toward the base.
Spores are ellipsoidal.
The spore print is brown, and the dimensions of the ellipsoid (oval-shaped) spores are 7.5–9 by 5–6 μm. The hymenium has cystidia both on the gill edge and face (cheilo- and pleurocystidia respectively), which are slender and filament-like, typically measuring 40–65 by 8–10.5 μm.