Oral splints are widely used in the treatment of myofascial pain of masticatory muscles, even though their mechanism of action is unknown. The present study evaluated the therapeutic efficacy of splints using a parallel, randomized, controlled and blind design. Following a sample size estimation, 63 subjects were recruited and assigned to 3 groups: (1) passive control: full occlusal splint worn only 30 min at each appointment: (2) active control: palatal splint worn 24 h/day: and (3) treatment: full occlusal splint worn 24 h/day. On each of 7 visits over 10 weeks, subjects rated on 100 mm visual analogue scales their pain intensity and unpleasantness at rest and after experimental mastication. The effect of pain on the quality of life was also rated on category scales. All pain ratings decreased significantly with time, and quality of life improved for all 3 groups. However, there were no significant differences between groups in any of the variables. These data suggest that the gradual reduction in the intensity and unpleasantness of myofascial pain, as well as the improvement of quality of life during the trial, was non-specific and not related to the type of treatment.