This study was designed to investigate the effects of injection with a local anesthetic agent or dry needling into a myofascial trigger point (TrP) of the upper trapezius muscle in 58 patients. Trigger point injections with 0.5% lidocaine were given to 26 patients (Group I), and dry needling was performed on TrPs in 15 patients (Group II). Local twitch responses (LTRs) were elicited during multiple needle insertions in both Groups I and II. In another 17 patients, no LTR was elicited during TrP injection with lidocaine (9 patients, group Ia) or dry needling (8 patients, group IIa). Improvement was assessed by measuring the subjective pain intensity, the pain threshold of the TrP and the range of motion of the cervical spine. Significant improvement occurred immediately after injection into the patients in both group I and group II. In Groups Ia and Ib, there was little change in pain, tenderness or tightness after injection. Within 2-8 h after injection or dry needling, soreness (different from patients' original myofascial pain) developed in 42% of the patients in group I and in 100% of the patients in group II. Patients treated with dry needling had postinjection soreness of significantly greater intensity and longer duration than those treated with lidocaine injection. The author concludes that it is essential to elicit LTRs during injection to obtain an immediately desirable effect. TrP injection with 0.5% lidocaine is recommended, because it reduces the intensity and duration of postinjection soreness compared with that produced by dry needling.