BACKGROUND AND PURPOSE Bleomycin (BLM), one of the most common sclerosants, is often used to treat venous malformations (VMs). The present study was designed to investigate whether endothelial mesenchymal transition (EndoMT) contributes to the therapeutic effects of BLM. EXPERIMENTAL APPROACH Endothelial and mesenchymal markers of HUVECs were measured by immunofluorescence, real-time quantitative PCR and Western blot analysis. Cell migration and tube formation assays were performed to evaluate endothelial cell function. Slug small-interfering RNA and specific inhibitors [Z-VAD-FMK for pan caspases, rapamycin for mammalian target of rapamycin (mTOR)] were used to investigate the mechanism. KEY RESULTS Long term (48 h or longer) treatment with BLM (0.1 mU·mL(-1) ) induced EndoMT in HUVECs, as manifested by a reduction in the expression of vascular endothelial-cadherin and an up-regulation in the expression of α-smooth muscle actin and fibroblast specific protein-1, as well as activation of the transcription factor Slug. The size and protein content of the transformed cells were increased. BLM also enhanced the migration of HUVECs but diminished their tube formation. By employing rapamycin, we demonstrated that activation of the mTOR pathway is involved in BLM-induced EndoMT in HUVECs. CONCLUSIONS AND IMPLICATIONS Our results show that a Slug-dependent EndoMT process is involved in BLM-induced therapeutic effects on endothelial cells and, more importantly, indicate the potential role of this process in the sclerotherapy of VMs.