Although pulmonary toxicity associated with gefitinib, an epidermal growth factor receptor inhibitor, has been reported recently, the accumulation of clinical information and the underlying mechanisms of gefitinib-induced interstitial lung disease (ILD) remain insufficient and unclear. After retrospectively reviewing the clinical records and chest X-rays of 489 lung cancer patients who were treated with gefitinib, we diagnosed four cases of gefitinib-induced ILD who underwent fiberoptic bronchoscopy and bronchoalveolar lavage (BAL). We found that the period of time from starting gefitinib to the onset of ILD was short, and concluded that a careful and close observation of a chest imaging study and a collection of respiratory symptoms was recommended. All four patients were treated with a high dose of corticosteroids, and ILD was resolved. We detected high levels of interferon-inducible protein-10 in BAL fluid, although we could not demonstrate the characteristic features of laboratory findings or BAL fluid cell analysis. We speculated that a Th1 type of lung tissue inflammation or lung injury might be involved as a part of mechanisms underlying gefitinib-induced ILD.