Over the past ten years, there has been an impressively growing number of reports about drug-induced pneumonitis (DIP) due to more than one hundred different drugs. The most troublesome question is how to establish with certainty the diagnosis. Usually, five criteria are necessary. 1) The administration of a drug on a more or less long term basis. 2) Newly occurrence of an interstitial pneumonitis (defined on symptomatology, radiological features, pulmonary function test results). 3) Elimination of all other causes of pneumonitis (haemodynamic, infectious, systemic, environmental diseases). 4) Broncho-alveolar lavage (BAL) cell data showing in most cases a lymphocyte alveolitis with an inverted CD4/CD8 ratio. In a certain number of ambiguous circumstances, coupling a provocation test with a sequentially performed BAL could firmly establish the diagnosis. 5) Rapid resolution within a few days or months of the pneumonitis as early as the incriminated drug administration is stopped. Nevertheless sometimes one or more of these criteria are not met, mainly when the pneumonitis is a fibrosis directly induced by a fibrosing toxic mechanism.