A 70-year-old man who had been a farmer presented to our hospital with a cough and shortness of breath. Partial pressure of arterial oxygen (PaO₂) was 71.2 Torr, and chest computed tomography showed centrilobular ground-glass opacities and consolidation. After admission, symptoms, PaO₂, and radiological findings showed improvement without drug treatment. Exposure of the patient to his house environment elicited no adverse response. However, an inhalation provocation test performed in the patient's hospital room using hay from his workplace resulted in recurrence of symptoms, reduction in PaO₂, elevation of white blood cell count and C-reactive protein level, reduction of vital capacity and diffusing capacity of the lung for carbon monoxide on pulmonary function testing, and development of infiltration as revealed by chest X-ray. We diagnosed him as having farmer's lung. Aspergillus niger was cultured from moldy hay, and the patient was positive for precipitating antibodies against this fungus. Precipitating antibodies against Thermoactinomyces vulgaris, Saccharopolyspora rectivirgula, Trichosporon asahii, Trichosporon mucoides, and pigeon serum were all negative. The performance of an inhalation provocation test in a hospital room using hay from the patient's workplace can be a useful method for establishing a diagnosis of farmer's lung.