In this open, uncontrolled trial, 20 patients with upper trapezius muscle trigger point pain and ipsilateral masseter muscle pain received a single trigger point injection of 2% lidocaine solution (without epinephrine) in the upper trapezius muscle. Following the trapezius injection, there was a significant (P < 0.001) reduction in pain intensity ratings for pain in the masseter region. In addition, there was a significant (P < 0.03) reduction in EMG activity in the masseter muscle. Overall, however, a significant relationship between EMG activity in the masseter and the self-report of pain was not found with the present data set. These clinical findings support the contention that sources of deep pain can produce heterotopic sensory and motor changes in distant anatomical regions.