Various types of treatment have been proposed for patients with temporomandibular joint (TMJ) osteoarthrosis and internal derangement. We examined the suitability of several assessment instruments developed in previous investigations for treatment-evaluation in a clinical trial comparing arthroscopic surgery with non-surgical treatment for a specific subgroup of TMJ osteoarthrosis and internal derangement, characterized by joint pain and restricted mobility. Twenty-one patients appeared to obey strict criteria for participation, and were randomly assigned to receive arthroscopic surgery followed by post-operative physical therapy (n = 9), or non-surgical treatment (n = 12). Individual outcome was assessed immediately following treatment and after 6 months based on a priori defined criteria. Patients treated with arthroscopic surgery reported more pain relief and more improvement of mobility than patients treated non-surgically, although clinical assessment results were comparable between the two groups. No major changes were observed regarding general well-being and interference with daily activities. The results of this pilot study indicate that the assessment methods are sufficiently sensitive for changes in pain, mobility, and functional ability. Despite the higher degree of subjective pain relief and appreciation of improvement of mobility in the arthroscopic surgery group, with these more formal assessment instruments this difference could not be confirmed. It is concluded that the preliminary results should be verified in a larger trial with a slightly adjusted design.