In this study, we investigated the fungi most frequently found in the air and the books of 28 libraries at the University of São Paulo, Brazil. In a second phase, 314 librarians were questioned about the presence of asthmatic or rhinitic symptoms, and the relationship with the site of work. Forty-nine percent of them reported this type of symptoms and 80% related them to the place of work. In the third stage, librarians underwent intracutaneous tests against the 20 fungi most frequently isolated in libraries. Eighteen librarians presented positive tests, 12 of whom reported rhinitis and 6 of whom were asymptomatic. In 19.5% of symptomatic patients wheals > 15 mm were observed after 6 h, while 9% of the asymptomatic patients presented similar wheals. Fourteen librarians with positive tests underwent tests with each of the 20 fungi constituting the pool. The airborne fungi isolated in libraries are likely to be found anywhere in the city of São Paulo, but they are likely to be present in higher concentrations in libraries, subsequently producing respiratory allergies.