Coincidentally, these toxic mushrooms resemble several edible species (most notably the straw mushroom) commonly consumed by humans, increasing the risk of accidental poisoning. A. phalloides is one of the most poisonous of all known toadstools. It has been involved in the majority of human deaths from mushroom poisoning, possibly including the deaths of Roman Emperor Claudius and Holy Roman Emperor Charles VI. It has been the subject of much research, and many of its biologically active agents have been isolated. The principal toxic constituent is α-amanitin, which damages the liver and kidneys, often fatally. There are only a few known antidotes, one of which is a compound derived from milk thistle.
The Death cap has a large and imposing epigeous (aboveground) fruiting body (basidiocarp), usually with a pileus (cap) from 5 to 15 cm (2–6 in) across, initially rounded and hemispherical, but flattening with age. The color of the cap can be pale-, yellowish-, or olive-green, often paler toward the margins and often paler after rain. The cap surface is sticky when wet and easily peeled, a troublesome feature, as that is allegedly a feature of edible fungi. The remains of the partial veil are seen as a skirtlike, floppy annulus usually about 1 to 1.5 cm (0.4–0.6 in) below the cap. The crowded white lamellae (gills) are free. The stipe is white with a scattering of grayish-olive scales and is 8 to 15 cm (3–6 in) long and 1 to 2 cm (3/8–3/4 in) thick, with a swollen, ragged, sac-like white volva (base). As the volva, which may be hidden by leaf litter, is a distinctive and diagnostic feature, it is important to remove some debris to check for it.
The smell has been described as initially faint and honey-sweet but strengthening over time to become overpowering, sickly-sweet and objectionable. Young specimens first emerge from the ground resembling a white egg covered by a universal veil, which then breaks, leaving the volva as a remnant. The spore print is white, a common feature of Amanita. The transparent spores are globular to egg-shaped, measure 8–10 μm (0.3–0.4 mil) long, and stain blue with iodine. The gills, on the other hand, are seen to stain pallid lilac or pink with concentrated sulfuric acid.