Adiaspiromycosis (ad"i-ah-spi"ro-mi-kósis) is a worldwide, noninfectious, nonarthropod transmitted fungal infection of lower vertebrates, most commonly rodents. Humans become an accidental host by inhaling dust-borne spores (conidia) of the saprophytic soil fungus, Emmonsia crescens (recently renamed Chrysosporium parvum variety crescens). We report 11 cases of this unusual deep mycosis from South America, Europe, and the United States. The severity of the disease depends on the number of spores inhaled. In limited inoculum, the disease remains localized (two cases), whereas in heavy inocula the fungus involves both lungs (nine cases) and presents as a diffuse reticulonodular infiltrate. In this disseminated form, patients usually complain of cough, dyspnea on exertion, and low-grade fever mimicking other systemic fungal infections and tuberculosis. It is difficult to unmask the fungus because it is not easily cultured nor is there a reliable serologic test. Therefore, a biopsy is required and the pathologist must recognize the large (ranging in size from 50 to 500 microns), round, Gomori methenamine-silver nitrate and periodic acid-Schiff reagent-positive spherules with a trilaminar wall. The spherules can be surrounded by either suppuration, epithelioid granulomas with or without necrosis, or concentric, hyalinized fibrosis. In the latter chronic stage, the organism may collapse, forming a variety of sizes and shapes thereby resembling other fungi, helminths, mineral particles, or inhaled pollen grains. Clinically, the infection most commonly regresses spontaneously, but may persist, or rarely progress, requiring surgical intervention with limited resection to attain cure.