purpose: To investigate the incidence of remote subclinical blastomycosis in a high-risk population of forestry workers. patients and methods: The study consisted of 39 male forestry workers from northern Minnesota and northern Wisconsin, areas endemic for blastomycosis but not for histoplasmosis. All subjects were histoplasmin skin test-negative, and none had ever been diagnosed with blastomycosis or pneumonia. An antigen-specific lymphocyte stimulation assay was performed to determine the presence of blastomycosis. results: Peripheral blood lymphocytes from 12 of 39 subjects demonstrated specific antigen-induced proliferation when stimulated with a purified alkali- and water-soluble antigen derived from the cell wall of Blastomyces dermatitidis. conclusion: The finding that 30% of these men had evidence of previous blastomycosis suggests that subclinical cases do occur sporadically, and are probably more common than diagnosed symptomatic cases. This is similar to histoplasmosis, in which the majority of infections are subclinical. However, the reservoir of persons with previous undiagnosed blastomycosis is probably small compared to the huge number of persons (perhaps 30 million) with prior histoplasmosis.