The long-term effectiveness of a prefabricated oral appliance (R) was compared with a stabilisation appliance (S) in patients with myofascial pain. Sixty-five patients diagnosed with myofascial pain at two centres for Stomatognathic Physiology in Sweden and Finland were included in a randomised controlled trial using Research Diagnostic Criteria for Temporomandibular Disorders, with history questionnaires and clinical examinations performed by blinded examiners at baseline and at 6- and 12-month follow-ups. Patients were randomly assigned either to the R or the S group. Treatment outcome was measured according to IMMPACT for four chronic pain outcome domains: pain intensity, overall improvement, physical functioning and emotional functioning. Physical functioning was classified for Graded Chronic Pain severities and assessed by the Jaw Functional Limitation scale. Emotional functioning composed of scores of non-specific physical symptoms and depression. There were no differences between groups at baseline. At both follow-ups, all outcome domains showed significant within-group improvement, without significant differences between groups. At 12 months, 72% of all patients reported a 30% reduction in worst pain and 63% of the patients a 50% reduction in worst pain. Overall improvement 'better' to 'symptom-free' was observed in 81% in the R and 64% in the S group at the 12-month follow-up. Graded Chronic Pain, Functional Limitation of the Jaw, non-specific physical symptoms and depression showed statistically significant reduction at 12-month follow-up. Results support the hypothesis that the effectiveness of the prefabricated appliance is similar to that of the stabilisation appliance in the long-term when treating patients with myofascial pain.