Although mammalian target of rapamycin inhibitors (mTORi) are used to treat various malignancies, they frequently induce active alveolitis and dyslipidemia. Abnormal lipid metabolism affects alveolar surfactant function and results in pulmonary disorders; however, the pathophysiology of lung injury and its relationship with lipid metabolism remain unknown. We investigated the relationship between lipid metabolism and alveolar epithelial injury, focusing on peroxisome proliferator-activated receptor-γ (PPAR-γ) as a lipid stress-related factor in mTORi-induced lung injury. We clinicopathologically examined three patients with mTORi-induced lung injury. We constructed an mTORi injury mouse model using temsirolimus in mice (30 mg/kg/day), with the vehicle control and bleomycin injury groups. We also constructed a cultured alveolar epithelial cell injury model using temsirolimus (0-40 μM) in the mouse lung epithelial cell line MLE-12 and performed analysis with or without pioglitazone (PPAR-γ agonist) treatment. All three patients had dyslipidemia and lung lesions of hyperplastic pneumocytes with foamy and enlarged changes. In the mouse model, temsirolimus induced significantly higher levels of total cholesterol and free fatty acids in serum and higher levels of surfactant protein D in serum and BAL fluid with an increase in inflammatory cytokines in the lung compared to control. Temsirolimus also induced hyperplastic foamy pneumocytes with increased lipid-associated spots and larger round electron-lucent bodies compared to the control or bleomycin groups in microscopic analyses. Multiple lipid-associated spots within the cytoplasm were also induced by temsirolimus administration in MLE-12 cells. Temsirolimus downregulated PPAR-γ expression in mouse lung and MLE-12 cells but upregulated cleaved caspase-3 in MLE-12 cells. Pioglitazone blocked the upregulated cleaved caspase-3 expression in MLE-12 cells. The pathogenesis of mTORi-induced lung disease may be involved in alveolar epithelial injury, via lipid metabolic stress associated with downregulated PPAR-γ expression. Focusing on the relationship between lipid metabolic stress and alveolar epithelial injury represents a potentially novel approach to the study of pulmonary damage.