Cap 5-15 cm broad, convex, becoming broadly convex, the disc typically depressed; margin striate, inrolled except in age, then decurved; surface finely pubescent, at maturity more or less glabrous, sticky in moist weather, dingy yellowish-brown to dull brown, bruising darker where handled; flesh thick, buff-brown, slowly darkening when cut; odor mild; taste sour.
Gills decurrent, close to crowded, narrow, easily peeled from the cap, intervenose, sometimes forming rudimentary tubes, cream-buff becoming buff-brown, bruising chestnut-brown.
Stipe 3-6 cm tall, 1-3 cm thick, equal or tapered to the base; surface smooth, pallid to dingy, yellowish-brown, bruising brown where handled, sometimes reticulate at the apex from anastomosing gills; flesh solid, darkening slowly when cut; veil absent.
Spores 7.5-10 x 4-6 µm, smooth, elliptical; spore print yellow-brown.
Scattered to gregarious in mixed hardwood/conifer woods; fairly common in the Bay Area under pines (Pinus); fruiting from late fall to mid-winter.
Although traditionally eaten in many parts of Europe, Paxillus involutus is now know to cause Immune Hemolytic Anemia. This anemia, which appears suddenly after years of consumption of the mushroom, can be fatal.
Paxillus involutus is a medium to large brown mushroom recognized by a broadly depressed cap, inrolled cap margin (from which the species name is derived), stout stipe, decurrent gills, and the tendency for all parts of the fruiting body to bruise brown. The gills which are easily peeled from the cap and sometimes form rudimentary tubes near the stipe are also distinctive. A related species, Paxillus atrotomentosus (=Tapinella atrotomentosa) also has a brownish cap but grows on wood, and has a dark-brown pubescent, eccentrically attached stipe.