The belfry and attic of a 100-year-old school building located in central Illinois were infested with a colony of big brown bats (Eptesicus fuscus). During the week of April 14, 1980, four workers disturbed the piles of bat droppings in the attic, causing dust to become airborne. Seven to 10 days later, all four workers developed symptoms and chest x-ray findings compatible with acute pulmonary histoplasmosis. Their sera had complement fixation (CF) titers of greater than or equal to 1:32 with fungal antigens and showed M and/or H bands by immunodiffusion tests. An additional 73 persons who had visited the building were also studied, leading to the finding of 16 additional cases of acute pulmonary histoplasmosis, identified on the basis of positive serologies and compatible symptoms. H. capsulatum was isolated from the sputum of one patient and from the soil beneath the hole in the building's eaves where the bats had been entering the attic. Cases were associated with exposure to the attic and with total hours of building exposure when compared with controls. The epidemic curve suggests that sporadic exposures occurred during the spring of 1980, with an epidemic occurring after the bat droppings were disturbed by the four workers.