Objectives: To investigate whether warm sterile water enhances the cytocidal effect of hypotonic shock on bladder cancer cells that show resistance to sterile water.Methods: Four bladder cancer cell lines of varying grades (T24, RT4, J82, and RT112) were exposed to sterile water, and morphological changes were closely observed under microscopy. Changes in cell membrane integrity and cell viability after water exposure were measured to determine the effects of water-induced hypotonic shock. Additionally, the effects of warm sterile water were analyzed.Results: T24, RT4, and J82 cells started swelling immediately upon exposure to water, followed by rupture within five minutes. RT112 cells demonstrated limited hypotonic swelling with significantly less cell rupture after 10 min. The percentages of viable cells at 10 min were 1.6 ± 0.8%, 3.5 ± 3.5%, 5.0 ± 3.2%, and 22.0 ± 10.3% for T24, RT4, J82, and RT112, respectively. The percentage of viable cells with 48 °C sterile water at one minute was 0% for RT112 cells.Conclusions: These findings support the efficacy of sterile water against bladder cancer cells and reveal that warm sterile water enhances the cytocidal effects of hypotonic shock, potentially avoiding the need for radical surgery.