Surface electromyograms (EMG) during maximal clenching and gum and peanut chewing were recorded from the temporal and masseter muscles of 30 patients with unilateral temporomandibular joint and myofascial pain-dysfunction (TMD). The gum-chewing integrated EMGs were transformed to a differential Lissajous EMG figure (DL-EMG) to facilitate comparison of mastication on the dysfunctional and opposite side. Variables, mainly those in the gum-chewing DL-EMG, were statistically analysed, and were compared with those of normal subjects. Patients were then classified into three types: (1) same side, (2) opposite side and (3) intermediate side. Same-side patients chewed peanuts on the dysfunctional side, although this side did not necessarily show abnormal behaviour during gum chewing. Opposite-side patients chewed peanuts by avoiding the dysfunctional side and showed excellent chewing activity on the side opposite the dysfunction. Intermediate-side patients had a chewing pattern that was a combination of the first two types. The relation between the pain side, the preferred or habitual mastication side, and the side showing better mastication in the EMG pattern was shown to be important in research on the origin of TMD syndromes. It was speculated that in the same-side patients, overwork on the side with the better dental condition for chewing was the main cause of the dysfunction, and that so-called hyperactivity on the dysfunctional side in TMD patients may result from habitual unilateral mastication.