Purpose: This study was designed to investigate the efficacy of arthrocentesis with and without injection of sodium hyaluronate (SH) into the upper joint space in the treatment of temporomandibular joint (TMJ) internal derangements. Patients and Methods: Forty-one TMJs in 5 males and 26 females aged 14 to 53 years comprised the study material. The patients' complaints were limited mouth opening, TMJ pain and tenderness, and joint noises during function. Patients were randomly divided into 2 groups in which only arthrocentesis was performed in 1 group and arthrocentesis plus intra-articular injection of sodium hyaluronate was performed in the other group. Both groups contained patients with disc displacement with reduction and with closed lock. Clinical evaluation of the patients was done before the procedure, immediately after the procedure, on postoperative day 1, and at 1, 2, 3, 4, 5, 6, 9, 12, 18, and 24 months postoperatively. Intensity of TMJ pain, jaw function, and clicking sounds in the TMJ were assessed using visual analog scales. Maximal mouth opening and lateral jaw movements also were recorded at each follow-up visit. Results: Both techniques increased maximal mouth opening, lateral movements, and function, while reducing TMJ pain and noise. Conclusions: Although patients benefitted from both techniques, arthrocentesis with injection of SH seemed to be superior to arthrocentesis alone. © 2001 American Association of Oral and Maxillofacial Surgeons