In a presumed common-source outbreak of blastomycosis 21 persons had extensive, repeated exposure. Eighteen of them gave evidence of infection by the fungus. Of the seven with symptoms three were severely ill, whereas the other four had clinically mild illness of short duration. An additional 11 asymptomatic persons were judged to have been infected on the basis of an abnormal chest roentgenogram or a positive blastomycin skin test or both. All patients recovered without specific antifungal therapy. Thus, it appears that the acute, epidemic form of blastomycosis can be a benign, self-limited illness. (N Engl J Med 290:540–543, 1974), FOR the two major mycotic diseases in the United States, histoplasmosis and coccidioidomycosis, the entire disease spectrum from acute self-limited pulmonary disease to the severe disseminated form is well described.