Animal tail has various functions, like balance, direction control, locomotion, etc. The coccyx (human's tail bone), which consists of three to five vertebral segments, does not protrude externally and does retain anatomical function to serve as a site of attachment of tendons and ligaments. Whether the human coccyx has any residual function of balance remains unknown. Coccydynia is pain in the region of coccyx and commonly results from trauma. PURPOSE: To evaluate the balance performance and flexibility of sit-to-reach before and after prolotherapy for the patients with coccydynia. METHODS: 8 patients (Age: 33.14 ±16.58 year-old; Body height: 162±10.97 cm, Body weight: 56.57±14.68 cm) who had coccydynia were included in this study. Prolotherapy as a therapeutic intervention was focused on para-sacrococcygeal maximal tendern points. 25 % glucose solution was injected I ml on each side, right and left. Balance performance and flexibility were evaluated before and one week after prolotherapy. Balance performance was determined by Smart Balance Master (Neurocom International Inc., USA), including Sensory Organization Test (SOT), Rhythmic Weight Shifting (RWS) and Limits of Stability(LOS). Flexibility of sit-to-reachas measured by HELMAS II physical fitness diagnosis system (SEWOO SYSTEM CO., LTD, USA). RESULTS: The equilibrium scores of the SOT were significantly improved in the conditions with eyes open, swayed support and fixed visual surround (81.92±8.53 vs. 89.33±4.96, p = 0.004); and with eyes open, swayed support and swayed visual surround (61.31±11.81 vs. 73.38±12.72, p = 0.020) one week after prolotherapy. In the LOS, the backward maximum excursion (35±19.68 vs. 27.25±14.40, p = 0.039) and the backward-left directional control (59.75±18.84 vs. 71.13±8.9, p = 0.031) were significantly increased. There was no significant difference in sitt-to-reach flexibilty. CONCLUSION: Balance performance of the patients with coccydynia was improved one week after prolotherapy, especially in more challenged condition of the SOT and in backward terget-reaching movement in the LOS. Human tail, the coccyx, seems to tune balance performance finely and determine the balance ccontrol, especially backward, as it gets hurt..