Aims: To investigate the time courses of myofascial temporomandibular disorder (TMD) pain and mandibular function impairment (MFI), and to identify predictive factors associated with these time courses. Methods: During a 12-month period following conservative TMD treatment, the time courses of myofascial TMD pain and pain-related disabilities were assessed by questionnaires. Ninety-six myofascial TMD patients participated, of whom 70 completed the study. Before treatment (baseline data), Characteristic Pain Intensity (CPI), MFI, parafunctional activities, and psychological status were assessed, and at completion of treatment and at 3, 6, 9, and 12 months, CPI and MFI were scored again. Individual time courses in scores were analyzed using linear growth modeling. Results: Baseline values of CPI had a positive correlation with CPI during follow-up (P = .002), whereas the influences of reported parafunctions and of pain elsewhere on CPI scores were close to significance (P = .058 and .06, respectively). Patients with a low somatization score showed a further decline in CPI during follow-up (P = .027), whereas patients with a high score showed a gradual increase (P = .030). Baseline values of MFI were positively correlated with MFI scores during the follow-up period (P = .000). The influence of reported parafunctions on MFI was not significant (P = .174), but that of pain elsewhere was (P = .004). The trend for a further decline in MFI values during follow-up was close to significance (P = .063) for patients with low somatization scores. Patients with high somatization scores showed a significant increase in MFI values (P = .007). Conclusion: Baseline reports of pain and impairment, oral parafunctional activities, pain elsewhere in the body, and somatization are associated with the severity and time course of myofascial TMD complaints following treatment. Keywords: mandibular function impairment, myofascial TMD pain, prognosis, time course