Extrinsic allergic alveolitis, with acute attacks of the disease, developed in a sawmill worker. The diagnosis was based on the history of exposure to moldy wood, on the symptoms and clinical signs, and on physiological and radiographic findings. A microbiological analysis of the ambient air of the patient's work environment showed that the air contained a considerable number of spores, both of fungi and actinomycetes. The most common fungal genus was Aspergillus. Thermoactinomyces vulgaris predominated the actinomycetal flora. The patient's serum contained precipitins to Thermoactinomyces vulgaris and Aspergillus fumigatus. Based on the microbiological and serological data, it was concluded that the causative agent(s) of the disease was either T vulgaris or species of the genus Aspergillus.