Objective: The purpose of this study was to evaluate the effects of the prolotherapy on the healing of the tendon tissue and the proliferation of fibroblasts in the injured Achilles tendons. Method: The tendons of twenty eight Sprague-Dawley rats were transected at 2 mm above the calcaneal insertion in the right Achilles tendon and sutured. Then they were allocated randomly into two groups: prolotherapy group and control group. We injected 20% dextrose 0.1 ml on injured tendon area of prolotherapy group immediately after transection. After 2 and 4 weeks, the diameters of tendons were measured on both the injured and uninjured tendon. The number of fibroblasts and the ratio of fibroblast to fibrocyte on the injured tendon tissues were measured by the image analyzer. Results: The diameters of the injured tendons of the prolotherapy group were not significantly different with that of the control group. However, the number of fibroblasts and the ratio of fibroblast to fibrocyte in the prolotherapy group showed significantly larger and more increased than the control group (p