We report a case of pneumonitis induced by tolfenamic acid. A 23-year-old woman was admitted to our hospital because of cough and fever. She had been treated with tolfenamic acid and other medications for lumbago. Nine days after the treatment, she developed erythema, fever, cough and dyspnea. Her chest X-ray revealed multiple patchy and micronodular shadows in both lung fields. Bronchoalveolar lavage fluid (BALF) showed increased total cells, lymphocytes, eosinophils and CD4/CD8 ratio. Microscopic examination of transbronchial lung biopsy (TBLB) specimens showed infiltration of mononuclear cells and eosinophils into the alveolar wall and the interstitium. After discontinuation of all drugs, her complaints, laboratory data and chest X-ray findings markedly improved. The lymphocyte stimulation test (LST) and challenge test for tolfenamic acid were positive. Based on these findings, we diagnosed this case as pneumonitis (eosinophilic pneumonia) due to tolfenamic acid. To our knowledge, there has been no reported case of pneumonitis due to this drug in Japan.