Background: The treatment outcome of temporomandibular disorders (TMD) can be assessed with Symptom Severity Index (SSI). In addition to this self-report measure, Patients’ Global Impression of Change (PGIC) provides information about the overall impression of change. The use of SSI and PGIC when assessing treatment outcome in the field of TMD, and their correlation, has not previously been reviewed. Aim: The aims of this study were to investigate the correlation between the scales SSI and PGIC, to gain knowledge about which factors that contribute to the impression of change and to identify the diagnoses that have the most improvement. Methods: Data from 193 patients was analyzed. The percentage change in symptoms between baseline and follow up was compared to PGIC. The patients were divided into three groups based on main diagnoses, for further analysis. PGIC categories “much improved” and “very much improved” were set as successful treatment outcome. Different factors impact on PGIC were analyzed.  Results: Majority of the patients had a successful treatment outcome. A higher mean percentage change correlated with a better PGIC value. Patients with articular related diagnoses needed a greater reduction in SSI to rate the impression of change as improved. According to the regression analysis, diagnosis group, age, number of treatment visits and SSI-baseline were associated with treatment outcome. Conclusions: The present study supports the use of both SSI and PGIC for assessment of the treatment outcome when treating TMD. However, a possible discrepancy between the scales addresses the need for further examination and change of treatment approach.