Methotrexate, a drug commonly used to treat juvenile idiopathic arthritis (JIA), has been reported to cause interstitial pneumonitis as a rare complication in adults with rheumatoid arthritis. Only 1 suspicious case of methotrexate pneumonitis in a child with JIA has been reported in 1998, though with no histopathologic proof. Given its rarity and nonspecific presenting symptoms, diagnosis may be challenging, and a life-threatening illness can occur without a high index of suspicion, as illustrated by this report of a 13-year-old girl with JIA who developed fever, nonproductive cough, and dyspnea as presenting features of interstitial pneumonitis after 1 year of methotrexate therapy. Chest high-resolution computed tomography revealed patchy ground-glass opacities and interlobular septal thickening without pleural effusion. Lung biopsy showed interstitial pneumonitis with diffuse alveolar damage. The symptoms and radiographs improved dramatically after withdrawal of methotrexate and administration of corticosteroids. A restrictive ventilatory defect with decreased diffusion capacity on pulmonary function testing persisted until 20 months after methotrexate withdrawal. There is no single pathognomic feature for methotrexate pneumonitis; rather, diagnosis relies on a constellation of clinical, radiologic, and pathologic findings. This report highlights the necessity for pediatricians to be continuously vigilant for interstitial pneumonitis in children receiving methotrexate who develop new unexplained pulmonary symptoms.