The aims of this study were to assess the thickness of the masseter muscle by means of ultrasonography and to investigate the relationship between masseter electromyographic activity and muscle thickness bilaterally, during maximum voluntary clenches. Participants in the study consisted of 52 young female adults (mean age 23·7 ± 2·5 years) without craniomandibular disorders and with full natural dentitions. The thickness of the masseter muscle was measured with a real-time ultrasound equipment. Electromyographic activity was recorded with bipolar surface electrodes, during maximum voluntary clenches. The error of the methods was calculated by double recordings in 15 subjects in a 4-week interval. The measurement error for the right muscle was 0·16 mm in thickness and 16·44 μV in electromyographic activity. For the left masseter the corresponding values were 0·19 mm and 18·01 μV. The relationship of masseter muscle thickness to its electromyographic activity was estimated by Pearson's correlation coefficient. The mean masseter thickness under contracted conditions was 13·9 ± 1·5 mm for the right side and 13·9 ± 1·4 mm for the left side. The mean maximum electromyographic activity was 379·0 ± 56·0 μV for the right muscle and 372·3 ± 73·2 μV for the left. Muscle thickness was strongly correlated to electromyographic maximum activity in the right masseter (r = 0·721, P ≤ 0·001) and moderately correlated in the left muscle (r = 0·407, P ≤ 0·01). The difference between the two sides is possibly because of the larger method error in the left side. It is apparent that ultrasonography can be used as a useful tool to assess masseter muscle functional capacity during full effort in healthy individuals.