Drug can cause various types of lung damages, with drug-induced pneumonitis (including acute interstitial pneumonia, usual interstitial pneumonia, desquamative interstitial pneumonia, nonspecific interstitial pneumonia, bronchiolitis obliterans with organizing pneumonia, eosinophilic pneumonia and hypersensitivity pneumonitis) being the most important among them. The incidence and the causative agents of drug induced pneumonitis have varied over time. Before 1980, anticancer agents and gold salts were the main drugs, and the number of causative drugs (61) and case reports was small. Recently, pneumonitis has increasingly been caused by Chinese herbal medicines, antibiotics, chemotherapy agents, anti-inflammatory drugs, analgesics, cytokines, and gold salts, and the number of case reports and drugs involved (177) has increased. Drug-induced pneumonitis has characteristics that depend on the causative agent. Review of our patients and reports in Japan revealed the following. Pneumonitis caused by anti-inflammatory drugs, analgesics, and antibiotics generally develops at 1-2 weeks after starting administration, and bronchoalveolar lavage and histologic examination of lung biopsies reveals the features of eosinophilic pneumonia. Such pneumonitis is associated with a high frequency of a positive drug lymphocyte stimulation test (DLST), and has a good outcome. Conversely, with pneumonitis caused by anticancer and immunosuppressive agents, the onset is often delayed and the disease has features of diffuse interstitial pneumonia and pulmonary fibrosis. The frequency of a positive DLST is low, and the outcome is generally poor. Pneumonitis induced by Chinese herbal medicines, gold salts, and antituberculosis agents has intermediate features between the above two types :i.e., it develops after 2-3 months or six months (gold salts), and resembles either eosinophilic pneumonia, BOOP or interstitial pneumonia. For in vitro identification of causative drugs, the DLST and the leukocyte migration inhibition test (LMIT) are generally used. The latter test is superior in sensitivity, suggesting that the mechanism of this test involves cytokines such as IL-1 alpha, IL-1 beta, IL-2, TNF-alpha, and IL-8.