The epidemiological and clinical aspects of Blastomycosis are reviewed. The central United States is the most heavily endemic area in the world, although the extent of the endemic zone has been mapped only by individual case finding, rather than by large skin test surveys (as was done for histoplasmosis). The difficulties in developing a sensitive and specific skin test antigen are reviewed, and the sequence of antigens from Blastomycin to antigen A to the ASWS (alkali and water soluble) antigen to the WI (Wisconsin) antigen are discussed. The absence of good immunological markers of remote subclinical disease means that the size of the iceberg of subclinical cases relative to clinically apparent and diagnosed pulmonary and extrapulmonary cases remains uncertain. Clinical presentations of blastomycosis range from (1) asymptomatic, currently discovered only in outbreak situation, (2) flulike illness of brief duration resembling other upper respiratory infections, (3) illness resembling bacterial pneumonia with acute onset, high fever, lobar infiltrates, and productive cough, (4) subacute or chronic respiratory illness with symptom complex resembling tuberculosis or lung cancer and radiographic presentation of fibronodular infiltrates or mass-like lesions, and (5) fulminant infectious adult respiratory distress syndrome (ARDS) with high fever, diffuse infiltrates, and progressive respiratory failure. Radiographic presentations are highly variable and even more confusing because of lack of standard terminology to describe these abnormalities. Examples of some of the radiographic presentations of blastomycosis are shown. Available information concerning computed tomographic studies is also reviewed. Special mention is made of blastomycosis in AIDS, which is uncommon but tends to be fulminant, systemic, and rapidly progressive. An overview of current diagnostic strategies and treatment options is also presented.