A growing body of epidemiologic information derives from the study of sporadic cases and seven epidemics of blastomycosis, although a complete epidemiologic description of the disease is impeded by the absence of a skin test and difficulty isolating the causative fungus, Blastomyces dermatitidis, from nature. Most cases occur sporadically in humans and dogs in a worldwide distribution. Endemic areas in the United States include southeastern, southcentral, and midwestern states, particularly Arkansas, Kentucky, Mississippi, and Wisconsin. Blastomycosis is acquired by inhalation of spores from nature and acute disease occurs 21 to 106 days (median 43 days) after exposure; cutaneous inoculation and sexual and intrauterine transmission are rare. Most human cases are in middle aged men with occupation-, or leisure-related activities involving the soil or wooded areas. Identification of a point source in nature at a beaver pond and lodge was first accomplished during a 1984 Wisconsin epidemic, advancing knowledge of the natural habitat and ecology of the fungus. Blastomcyces alkali- and water-soluble antigen, utilized for in vitro lymphocyte transformation studies during the epidemic, may hold considerable promise for incorporation into a skin test.