This study analysed the prognostic factors for successful arthrocentesis with and without sodium hyaluronate (SH) injection for the treatment of temporomandibular joint (TMJ) disc displacement without reduction (DDwoR) using clinical and radiological results. 29 TMJs in 25 patients with DDwoR were included. Patients were treated with arthrocentesis or arthrocentesis followed by intra-articular (i.a.) injection of SH. Treatment was evaluated for postoperative range of maximum mouth opening and the degree of postoperative pain on a VAS. Prognostic factors analysed were age, sex, duration of locking, trauma history, previous TMJ treatment, depression, bruxism, malocclusion and missing teeth. Degenerative changes were evaluated as probable prognostic factors. After treatment, 24 joints (83%) fulfilled the criteria for success. Duration of locking and present preoperative degenerative changes were the most significant factors for treatment outcome. The results suggest it is sufficient to use only arthrocentesis in patients without preoperative degenerative changes and arthrocentesis with SH in patients with degenerative changes on their preoperative MRIs, but because there were some significant differences between the two groups preventing the authors from comparing them statistically, they cannot come to that conclusion. To clarify the use of SH in such cases, standardized study groups are necessary for future studies.