A 59-year-old woman receiving methotrexate and tacrolimus for rheumatoid arthritis (RA) was referred to our hospital following bilateral ground-glass opacity observed in her chest X-ray and elevated serum KL-6. After methotrexate and tacrolimus cessation, shortness of breath developed and ground-glass opacity observed in the chest computed tomography rapidly worsened. Bronchoalveolar lavage showed increased lymphocytes, and trans-bronchial lung biopsy confirmed lymphocytic alveolitis. In addition, the patient had serum antibodies against Trichosporon asahii, a fungal pathogen. The mildew in her bathroom and washing machine which were the source of the fungus were removed, which resulted in no further relapse of the condition. In this patient's case, methotrexate and tacrolimus may have masked and suppressed summer-type hypersensitivity pneumonitis.
Keywords: KL‐6; Methotrexate; rheumatoid arthritis; summer‐type hypersensitivity pneumonitis; tacrolimus.