A farmer and his son, who treated straw in a cowshed, were admitted to our hospital because of severe dyspnea during summer time. Their chest X-ray films revealed bilateral reticulonodular shadows in the middle to lower lung fields. Bronchoalveolar lavage (BAL) fluid analyses showed a high proportion of lymphocytes and an increased CD4/8 ratio. They were diagnosed with farmer's lung and treated with pulse therapy with methylprednisolone and tapering of steroid. Hypoxemia and interstitial shadow improved, though the farmer relapsed one day after getting home. Immune precipitation showed positive reactions against Aspergillus fumigatus, Aspergillus terreus and Nocardiopsis alba. Their beta D-glucan levels in BAL fluid were higher than those of healthy normal volunteers, whereas their beta D-glucan levels in serum were below the detection levels. Ventilation of the cowshed and wearing a mask should prevent recurrence of the disease.