It is widely distributed across the Northern Hemisphere and has been accidentally introduced to Australia, New Zealand, and South America; it is likely to have been transported in soil of European trees to those countries. Various shades of brown in color, the fruiting body resembles a brown wooden top and may be found in deciduous and coniferous woods, and grassy areas in later summer and autumn. The cap bears a distinctive inrolled rim and decurrent gills which 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.
Although it has gills rather than pores, it is has long been recognized as belonging to the pored mushrooms of the Boletales; French mycologist René Maire had erected the family Paxillaceae in 1902, placing it between the agarics and boletes and recognizing the groups' similarities with the latter group. More recent molecular research confirms its relations; the genus Gyrodon, with the decurrent-pored mushroom G. lividus, is the closest group to the genus Paxillus and together they lie near the base of a tree from which the genus Boletus arises.
Its brownish color and funnel-like shape could lead to its confusion with several species of Lactarius, though many of these have some degree of toxicity themselves.
Paxillus involutus forms ectomycorrhizal relationships with a number of coniferous and deciduous tree species, with fruiting bodies appearing in late summer and autumn. It is more common in coniferous woods in Europe, where it may be locally abundant in some areas. Within woodland, it prefers wet places or boggy ground, and avoids calcareous (chalky) soils. It appears to grow in similar places to Boletus badius. It is found in both deciduous and coniferous woodland in North America, and is common under plantings of white birch (Betula papyrifera) in urban areas in the Pacific Northwest. It can be found growing on lawns and old meadows throughout its distribution. Fruiting bodies are generally terrestrial, though may be found on woody material around tree stumps.
Australian mycologist John Burton Cleland noted it occurring under larch (Larix), oak, pine, birch and other introduced trees in South Australia in 1934, and it has subsequently been recorded in New South Wales, Victoria, and Western Australia. It has been recorded under introduced birch (Betula) and hazel (Corylus) in New Zealand. Mycologist Rolf Singer reported a similar situation in South America, with the species recorded under introduced trees in Chile. It is likely to have been transported in soil of European trees to those countries.
This relatively rare immunohemolytic syndrome occurs following the repeated ingestion of Paxillus mushrooms. Most commonly it arises when the person has ingested the mushroom for a long period of time, sometimes for many years, and has shown mild gastrointestinal symptoms on previous occasions. The Paxillus syndrome is better classed as an allergic reaction than a toxicological reaction as is not caused by a genuinely poisonous substance but by the antigen in the mushroom. This antigen is still of unknown structure but it stimulates the formation of IgG antibodies in the blood serum. In the course of subsequent meals, antigen-antibody complexes are formed; these complexes attached to the surface of blood cells leading to their lysis.
The symptoms from poisoning are rapid in onset; consisting initially of vomiting, diarrhea, abdominal pain, and associated hypovolemia. Shortly after these initial symptoms appear hemolysis develops signified by oliguria, anuria, hemoglobin in the urine (hemoglobinuria) and anemia. Medical laboratory tests consist of increasing bilirubin and free hemoglobin, and falling haptoglobins. Hemolysis may lead to numerous complications including acute renal failure, shock, acute respiratory failure, and disseminated intravascular coagulation. These complications can cause significant morbidity with fatalities having been reported.
There is no antidote for poisoning, treatment consists of monitoring complete blood count, renal function, blood pressure, and fluid and electrolyte balance and correcting any abnormalities. The use of corticosteroids may be a useful adjunct in treatment; corticosteroids protect blood cells against hemolysis thereby reducing complications. Plasmapheresis may also be beneficial in improving outcome. Plasmapheresis reduces the circulating immune complexes in the blood which cause the hemolysis, removing these complexes with extracorporeal techniques such as plasmaphoresis can then reduce the immune hemolysis. Additionally hemodialysis can be used as a supportive treatment for patients with compromised renal function or renal failure.