Drug-induced interstitial lung disease (ILD), particularly pulmonary fibrosis, is of serious clinical concern. Gefitinib, a tyrosine kinase inhibitor of the epidermal growth factor receptor (EGFR), is beneficial as a drug for treating non-small cell lung cancer; however, this drug induces ILD and the molecular mechanisms underpinning this condition remain unclear. We recently reported that expression of heat shock protein 70 (HSP70) protects against bleomycin-induced pulmonary fibrosis, an animal model of pulmonary fibrosis. In this study, we have examined the effects of drugs known to induce ILD clinically on the expression of HSP70 in cultured lung epithelial cells and have found that gefitinib has a suppressive effect. Results of a luciferase reporter assay, pulse-labelling analysis of protein and experiments using an inhibitor of translation or transcription suggest that gefitinib suppresses the expression of HSP70 at the level of translation. Furthermore, the results of experiments with siRNA for Dicer1, an enzyme responsible for synthesis of microRNA, and real-time RT-PCR analysis suggest that some microRNAs are involved in the gefitinib-induced translational inhibition of HSP70. Mutations in the EGFR affect the concentration of gefitinib required for suppressing the expression of HSP70. These results suggest that gefitinib suppresses the translation of HSP70 through an EGFR- and microRNA-mediated mechanism. In vivo, while oral administration of gefitinib suppressed the pulmonary expression of HSP70 and exacerbated bleomycin-induced pulmonary fibrosis in wild-type mice, these effects were not as distinct in transgenic mice expressing HSP70. Furthermore, oral co-administration of geranylgeranylacetone (GGA), an inducer of HSP70, suppressed gefitinib-induced exacerbation of bleomycin-induced pulmonary fibrosis. Taken together, these findings suggest that gefitinib-induced exacerbation of bleomycin-induced pulmonary fibrosis is mediated by suppression of pulmonary expression of HSP70 and that an inducer of HSP70 expression, such as GGA, may be therapeutically beneficial for the treatment of gefitinib-induced pulmonary fibrosis.