Background Circulating mesenchymal stem cells contribute to bone repair. Their incorporation in fracture callus is correlated to their bioavailability. In addition, Granulocyte-colony stimulating factor induces the release of vascular and mesenchymal progenitors. We hypothesized that this glycoprotein stimulates fracture healing, and analyzed the effects of its administration at low doses on bone healing. Methods 27 adult male Sprague-Dawley rats underwent mid-femur osteotomy stabilized by centromedullar pinning. In a post (pre) operative group, rats were subcutaneously injected with 5µg/kg per day of Granulocyte-colony stimulating factor for 5 days after (before) surgery. In a control group, rats were injected with saline solution for 5 days immediately after surgery. A radiographic consolidation score was calculated. At day 35, femurs were studied histologically and underwent biomechanical tests. Findings 5 weeks after surgery, mean radiographic scores were significantly higher in the Preop group 7.75 (SD 0.42) and in the Postop group 7.67 (SD 0.52) than in the control group 6.75 (SD 0.69). Biomechanical tests showed femur stiffness to be more than three times higher in both the Preop 109.24N/mm (SD 51.86) and Postop groups 100.05N/mm (SD 60.24) than in control 32.01N/mm (SD 15.78). Mean maximal failure force was twice as high in the Preop group 68.66 N (SD 27.78) as in the control group 34.21N (SD 11.79). Histological results indicated a later consolidation process in control than in treated groups. Interpretation Granulocyte-colony stimulating factor injections strongly stimulated early femur fracture healing, indicating its potential utility in human clinical situations such as programmed osteotomy and fracture.