Four workers, the total work force employed at a Shiitake farm, developed cough and sputum production following a variable period of exposure to Shiitake mushrooms. All four had abnormal diffusing capacity and three had abnormal spirometry values. Chest roentgenograms demonstrated an interstitial pattern in one worker. Pulmonary function tests performed before and during several days of work demonstrated a significant decrease (> 20%) in forced vital capacity (FVC) and/or maximal midexpiratory flow (MMEF) in three workers. Although specific antibodies to an extract of Shiitake spores were detected in sera from three workers none were IgE. High levels of Shiitake spores were detected in growing rooms (> 106/m3) as well as other locations at the farm. Shiitake spore airborne antigen, detected by an immunochemical assay, was present in dust collected with a volumetric sampler from different locations at the farm. Antigenic determinants of Shiitake spore antigens, in common with antigens from other cultivated mushrooms (Agaricus and Pleurotus) were demonstrated by ELISA inhibition assay. This study demonstrates that workers exposed to high levels of Shiitake spores develop symptoms and laboratory findings suggestive of hypersensitivity pneumonitis (HP). Strict environmental control and the wearing of a face mask is probably needed to reduce the high risk of sensitization and possible development of immunological lung disease. Shiitake spores must be considered as an aetiological agent of mushroom workers' lung.