Purpose: Due to unsatisfactory success in the treatment of local and systemic bladder cancer and the low response rates to commonly used chemotherapy (CT) alternative and additive approaches must be found. The function of vascular endothelial growth factor (VEGF) in neo-angiogenesis and, therefore, in solid tumors makes it a promising target for a specific antitumor therapy. We investigated the possibility of sensitizing transitional bladder cancer cell lines to CT by pretreatment with VEGF antisense (AS) oligodeoxynucleotides (AS-ODNs). Materials and Methods: The human bladder cancer cell lines EJ28 and 5637 were transiently transfected with 3 antiVEGF AS-ODNs, followed by incubation with 3 doses of mitomycin C, gemcitabine or cisplatiri CT. WST-1 (a sodium salt of 4-[3-(4-iodophenyl)-2-(4-nitrophenyl)-2H-5-tetrazolio]-1,3- benzene disulfonate) assay (Roche, Mannheim, Germany) was performed to assess effects on cell viability. Apoptosis was examined by Annexin V staining. In all experiments a nonsense ODN served as a control. Results: Each cell line responded in a dose dependent manner to all CTs. Combined treatment with VEGF AS-ODNs and CT resulted in decreased viability compared with isolated CT. VEGF857 plus CT significantly decreased the viability of the 2 cell lines compared with nonsense ODN plus CT for all 3 CT agents (p < 0.007). This detected chemosensitization was based on an AS mediated increase in apoptosis. Conclusions: One of the 3 AS-ODNs tested (VEGF857) significantly sensitizes human transitional cell carcinoma cells to CT. We suggest VEGF as an additional putative target to enhance the therapeutic benefit of, for example mitomycin C and gemcitabine instillation treatment schedules.