Twelve cases of possible hypersensitivity pneumonitis in a centrally located office zone of an eight floor building were studied. The cases occurred in a location which had been flooded several times due to plumbing problems in an overhead cafeteria. The workers presented with cough, muscle aches, nausea, chills, fever, and fatigue that occurred during the work week and disappeared on weekends. Evaluation included an industrial hygiene survey, personal history questionnaires, pre and post shift spirometry, serum precipitin tests, and chest x-rays. Airborne microorganisms were assessed in air samples and samples of carpets, ceiling tiles, drain pipes, fan coils, and filters. The twelve cases were compared to 96 persons not affected. Significant differences in lung function tests between the cases and the noncases were observed for the mean single breath carbon-monoxide diffusing capacity but not for lung volume curves. No significant differences were determined between cases and noncases for the presence of precipitating antibodies to microorganisms isolated from the environmental survey. The authors conclude that the mean single breath carbon monoxide diffusing capacity is an appropriate cross sectional test for field studies of building associated lung disease.