The aim of the study was to evaluate the effects that an anterior cruciate ligament injury of the knee has on postural control and activity of neck, head and trunk muscles to investigate the existence of connections between the masticatory system and body posture. Surface electromyographic activity of the muscles at mandibular rest position, and during maximal voluntary clenching and posturometric and stabilometric measurements of 25 adult patients having pathology on the left knee were compared with a control non-pathological group. At rest, the patients showed a higher muscular activity of anterior temporalis, masseter, sternocleidomastoid and lower trapezius, compared with the control subjects (P < 0·05). At maximal voluntary clenching, the patients showed a lower muscular activity of the right anterior temporalis and masseter and a higher muscular activity of the lower trapezius, compared with the control subjects. For the stabilometric measurements, all the subjects showed a significant reduction in the postural centre of pressure path length during the test with eyes open and cotton rolls, compared with the test with eyes closed and mandibular rest position (P < 0·05). In addition, the patients showed a significant displacement of the postural centre of pressure in a forward direction (P < 0·05) and into the right side (P < 0·05), compared with the control subjects. Anterior cruciate ligament injury appears to be associated to a change in the activity of head, neck and trunk muscles and to a change in the position of the postural centre of pressure. Cotton rolls seem to improve the stability of the subject.