A 70-year-old man had for two months taken a dietary supplement containing Agaricus as followup after an operation for lung cancer. He was admitted to the hospital because of fever. Although he was diagnosed as pneumonia and treated by antibiotic, his symptoms did not improve. A chest CT scan revealed ground-glass attenuations with some small patchy consolidations in the left lung, and increasing total cell counts and lymphocytes in bronchoalveolar lavage fluid were observed. Because the results of a drug lymphocyte stimulation test against Agaricus were positive, we diagnosed him as drug-induced pneumonitis associated with Agaricus. He was treated with oral corticosteroid, and his symptoms and lung abnormalities improved. To our knowledge, few cases of lung injury induced by Agaricus have been reported. We need to consider that many patients with cancer take dietary supplements.