''Mycena leptocephala'', commonly known as the nitrous bonnet, is a species of fungus in the Mycenaceae family. The mushrooms have conical grayish caps that reach up to 3 cm in diameter, and thin fragile stems up to 5 cm long. The gills are gray and distantly spaced. The spores are elliptical, typically measure 7–10 by 4–6 μm, and are white in deposit. When viewed under a light microscope, the gills have abundant spindle-shaped cystidia on the gill edges, but few on the gill faces. The mushroom is found in North America and Asia, where it grows singly or in groups on conifer needles, cones and sticks on the forest floor. It has a distinctive odor of bleach; the edibility is unknown. Similar species include ''Mycena alcalina'', ''M. austera'', and ''M. brevipes''.