OBJECTIVE: To compare the ultrasonographic findings of chronic lateral epicondylitis of elbow with partial tear of common extensor tendon before and after prolotherapy. METHOD: The subjects were 12 cases of chronic lateral epicondylitis with partial tear of common extensor tendon, which were confirmed by ultrasonography. We examined the common extensor tendon with ultrasonography at initial visit. After injecting 15% dextrose solution monthly for five times, follow up ultrasonography was performed one month after last injection. RESULTS: Before prolotherapy, every case had anechoic focus without normal fibrillar pattern, which represented partial tear of tendon. Seven cases showed focal or diffuse hypoechoic foci with loss of normal fibrillar pattern of tendon, which represented tendinosis. After prolotherapy, initial anechoic foci were changed to smaller size with diffuse fibrillar pattern inside in 6 cases, to the same the sized hypoechoic foci with diffuse fibrillar pattern inside in 2 cases. And a few fibrillar pattern were seen within the initial anechoic focus in 1 case and most of anechoic foci were filled with fibrillar pattern except small anechoic foci in 3 cases. CONCLUSION: Prolotherapy can help the recovery process of chronic lateral epicondylitis. And ultrasonography can be a useful method to evaluate the therapentic effect of lateral epicondylitis.