The clinical and epidemiologic characteristics of chronic pulmonary histoplasmosis, as compared with the more familiar acute pulmonary histoplasmosis, are relatively unknown. Opinions vary as to the pathogenesis, and only the severe forms of the disease are readily recognized. Over a 22 month period following the excavation of blackbird roost, an unusual outbreak of chronic pulmonary histoplasmosis occurred in a town in southern Kentucky. Thirteen of the cases developed over a span of only four months. An associated outbreak of acute pulmonary histoplasmosis did not occur. During the course of the ensuing investigation, the residential addresses of the affected persons were noted to be clustered about the previously excavated blackbird roost. A case-control study was initiated; the median distance of the residential addresses of the affected persons was found to be 1.0 miles from the roost, compared with 3.2 miles for the control subjects (P less than 0.001). It was concluded that (1) the excavated blackbird roost had served as the common source of the epidemic; (2) the inhalation of exogenous spores accounted for the infections; (3) the spectrum of clinical illness ranged from asymptomatic and mild illness to cavitary disease with considerable morbidity; and (4) following excavation, blackbird roosts may remain an infection hazard for an indefinite period of time.