Blastomycosis is a systemic pyogranulomatous disease caused by the thermally dimorphic fungus Blastomyces dermatitidis. The disease is endemic to parts of the midwestern and southcentral United States and Canada, although blastomycosis has been reported worldwide, including isolated reports from Africa and Central and South America. Within the United States and Canada, the disease is concentrated in areas along the Mississippi and Ohio River basins and the Great Lakes. In endemic areas, small point-source outbreaks of blastomycosis have been associated with recreational and occupational activities occurring in wooded areas along waterways. Current evidence indicates that B. dermatitidis exists in warm moist soil enriched by organic debris, including decaying vegetation and wood. Most infections with B. dermatitidis occur through inhalation of aerosolized spores, although infection through direct inoculation has been reported rarely. Primary infections are usually asymptomatic or may result in a self-limited flulike illness. Hematogenous dissemination of organisms from the lung can result in extrapulmonary manifestations. Blastomycosis is usually recognized as a chronic, indolent systemic fungal infection associated with various pulmonary and extrapulmonary manifestations. Pulmonary blastomycosis usually manifests as a chronic pneumonia syndrome characterized by productive cough, chest pain, hemoptysis, weight loss, and low-grade fever. There are no distinguishing radiologic features of pulmonary blastomycosis, although one or more fibronodular infiltrates or mass lesions with or without cavitation are common, often mimicking other granulomatous diseases or bronchogenic carcinoma. Hilar adenopathy and pleural effusions are uncommon. Rarely, diffuse pulmonary infiltrates consistent with adult respiratory distress syndrome may occur secondary to blastomycosis.