Aims: To determine whether subjects with temporomandibular disorders (TMD) manifesting as chronic myofascial pain (MFP) involving the masseter muscle present with significantly greater masseter muscle width, as evidenced by ultrasound, compared to individuals without MFP. Methods: A case-control study was carried out. A total of 31 subjects presenting with MFP of the masticatory muscles involving the masseter muscle and 35 controls with TMD but no diagnosis of MFP, matched by age and sex, were included. Ultrasound was used to measure the maximum width of both masseter muscles at the intermediate point between the origin and insertion of the muscle in the light occlusal contact (LOC) position and under maximum contraction. Each side was analyzed separately. Means were compared by using single-factor analysis of variance and Mann-Whitney U test; P .05 was considered to reflect statistical significance. Results: In the study group, the right masseter muscle had a mean ± standard deviation width of 8.6 ± 1.8 mm under LOC (controls: 8.6 ± 1.6 mm; P = .85) and 11.5 ± 2.1 mm under maximum contraction (controls: 11.7 ± 1.9 mm; P = .86). The analagous measures in the left masseter muscle were 8.6 ± 1.6 mm under LOC (controls: 8.2 ± 1.5 mm; P = .42) and 11.3 ± 1.8 mm under maximum contraction (controls: 11.5 ± 1.8 mm) (P = .79), respectively. The increase in width of the right masseter muscle was 2.9 ± 2.1 mm (controls: 3.1 ± 1.2 mm; P = .67) in absolute terms and 1.4 ± 0.3 mm (controls: 1.4 ± 0.2 mm; P = .91) in relative values (width at maximum contraction/LOC width). In the case of the left masseter muscle, the respective values were 2.8 ± 1.7 mm (controls: 3.2 ± 0.9 mm; P = .25) and 1.3 ± 0.2 mm (controls: 1.4 ± 0.1 mm; P = .32). Conclusion: There were no statistically significant differences in masseter muscle width between MFP subjects and control subjects under LOC conditions or maximum contraction. The increase in width under maximum contraction was likewise not significantly different between the groups. Schlagwörter: chronic pain, masseter, myofascial pain, ultrasound