Photobiomodulation (PBM) has been studied to modify the cellular response to ionizing radiation. However, its combination with radiotherapy (RT) has not been reported in cancer treatment. The aim of this study was to evaluate the effects of PBM applied before RT on breast tumor-bearing mice. Female BALB/c mice were inoculated with breast 4T1 cells into mammary fat pad and divided into 4 groups (n =5 per group): control (with no treatment), only RT, and PBM combined to RT in two different protocols. RT was locally applied using a 60Co source with dose of 60 Gy in fractions of 15 Gy. For PBM, a red laser (660 nm, 500 mW/cm2) was used in two regimes: single exposure 24 h before RT (fluence of 150 J/cm2) and immediately before each RT session (fluence of 37.5 J/cm2 per session). After treatment, tumor volume, platelets, white and red blood cell levels were evaluated during 14 days. Our results showed no statistically significant differen ces in t umor volume, platelet and red blood cell levels comparing control, RT and PBM+RT groups. However, PBM was able to sustain normal white blood cell levels compared to RT and control groups. In addition, mice that received PBM concomitant with RT presented a longer survival. In fact, for this group only 12.5 % of the animals died during experimental period. These findings indicate that PBM could be combined to RT to provide therapeutic anti-cancer benefits.