To explore the possibility of using blue light for extracorporeal circulation therapy in patients with leukemia, the effects of blue light on cell growth in vitro and in extracorporeally circulated blood of rats with leukemia were evaluated. When HL60 cells circulated extracorporeally using a peristaltic pump were exposed to blue light for 5 h, the growth of the cells was found to be markedly suppressed. Then, the blood of rats with erythroblastic leukemia, induced by the administration of tap water containing l-ethyl-l-nitrosourea (ENU) for 9-16 weeks, was circulated extracorporeally and exposed to blue light for 3 h. Lymphocytes were separated from the peripheral blood immediately after the end of blue-light-exposure and incubated for 7 days. The growth of leukemic cells was found to be significantly suppressed following exposure to blue light, whereas the growth of normal lymphocytes was unaffected. These findings suggest that cancer cells may be more sensitive to blue light than normal cells.