A prospective evaluation of the effectiveness of temporomandibular joint arthroscopy. This is a prospective study to evaluate therapeutic arthroscopy for internal derangement of the temporomandibular joint (TMJ). Fifty-nine patients with 76 abnormal joints were evaluated preoperatively for pain, noise, maximal incisal opening (MIO), and deviation on opening. Preoperative and postoperative magnetic resonance imaging (MRI) scans were obtained for 29 joints. Patients were treated by superior joint arthroscopy, lysis of adhesions, lavage, and steroid injection, along with preoperative and postoperative splint and physiotherapy. Pain, noise, and motion were evaluated at three time periods: 1) early (10 to 30 days); 2) intermediate (1 to 6 months); and 3) late (greater than 6 months). At early, intermediate, and late follow-up, increase in MIO was statistically significant (P less than .05). Noise did not return in the majority of patients. Disc position, evaluated by MRI, did not appear to change in 25 of 29 joints and did not correlate with clinical outcome. The results of this study indicate that TMJ arthroscopy is effective in reducing pain and increasing motion in patients with TMJ internal derangement. 