Bakers are exposed daily to flour and may be susceptible to immunologic occupational diseases. A 30-year-old, nonsmoking, female baker was referred for progressive dyspnea on exertion, basal crackles on auscultation, restrictive lung function, decreased diffusing capacity of the lung for carbon monoxide, ground glass hyperdensities with a mosaic pattern on high-resolution CT scan, 25% lymphocytosis by BAL, and cellular chronic bronchiolitis with peribronchiolar interstitial inflammation by lung biopsy specimen. Cultures from flours isolated nine species, including Aspergillus fumigatus. Twenty-six antigens were tested. Serum-specific precipitins were found against A fumigatus, the flour mite Acarus siro, and total extracts from maize and oat. Outcome was favorable with cessation of occupational exposure to flours and transient therapy with prednisone and immunosuppressive agents. To our knowledge, this report is the first of a well-documented case of hypersensitivity pneumonitis due to sensitization to fungi- and mite-contaminated flours. Hypersensitivity pneumonitis--and not only asthma and allergic rhinitis--should be suspected in bakers with respiratory symptoms.