Aims: To determine via a prospective investigation whether the presence of neuropsychologic or cognitive deficiencies could be identified in patients with temporomandibular disorders (TMD) and used to predict treatment outcome. This was based on the theory that measurable reductions in neuropsychologic and cognitive function might have a negative impact on treatment outcome in patients with essentially nontraumatic TMD, as has been shown for patients with posttraumatic TMD. Methods: Various neuropsychologic, psychosocial, and clinical parameters (including but not limited to the Peterson-Peterson Consonant Trigram Test and the California Verbal Learning Test) were used to pretest patients suffering from TMD prior to treatment. Patients were then entered into treatment, after which determination of treatment success was made both by the use of visual analog scales for pain and global transitional outcome measures (eg, “better,” responders versus “same/worse,” nonresponders). After determination of treatment success was made, treatment response was correlated with the various clinical, cognitive, and neuropsychologic test scores. Results: Overall, the nonresponders did worse in both the neuropsychologic and psychosocial assessments, with greater memory deficits, sleep disturbances, depression, and fatigue and lower energy levels as compared to responders. Among the best predictors of treatment outcome were the Peterson- Peterson Consonant Trigram Test scores, as well as the scores on the California Verbal Learning Test (ie, poorer test outcomes predicted nonresponse). Neither responders nor nonresponders could be distinguished from one another based on clinical parameters of maximum interincisal opening or muscle tenderness. Three psychosocial variables were also found to be predictors of poor outcome: sleep disturbance, fatigue, and income. Pretreatment pain on chewing was also found to be a reliable predictor of poor treatment outcome. Conclusion: We conclude that various neuropsychologic, psychosocial, and some clinical parameters may provide pretreatment prediction of treatment outcome in an idiopathic TMD population.