Associations of evening and morning masticatory muscle pain and nocturnal electromyography (EMG) activity with psycho-behavioural factors and occlusal splint therapy were studied during a 20-week study-protocol. Over a period of almost 2 years, only eight of the 120 eligible patients were willing to enroll the study protocol. Further, four of the eight participants dropped out during the study, and approximately 20-30% of the nocturnal EMG recordings failed. Because of the impractical and unworkable nature of the protocol, the study was prematurely terminated and the results of the four remaining individuals are reported here as single-patient clinical trials. Univariate and multiple regression analyses revealed that in three of the four patients, changes in nocturnal EMG activity were associated with the period of splint therapy. However, no associations were found between the changes in nocturnal EMG activity and the observed changes in muscle pain. In two patients, the changes in muscle pain were associated with the period of splint therapy and with the changes in psychological stress. Within the limitations of single-patient clinical trials, it can be concluded that changes in chronic masticatory muscle pain seem to be more related to changes in psychological stress than to those in parafunctional activities.