Muscle afferent block (MAB) is an intramuscular injection of 0.5% lidocaine and pure ethanol with a volume ratio of 10:1, introduced as an alternative to botulinum toxin injection for focal dystonia and spasticity. As in the case of botulinum toxin injection, the precise localization of target muscles is crucial to obtain the maximal effect from MAB. For this purpose, we performed ultrasonography of cervical muscles (echomyography) in 20 patients with cervical dystonia (11 men, 9 women; mean age 46.1), with ultrasonograph SSD-5500 (Aloca Co. Ltd., Japan) and a 7.5 MHz linear probe. In untreated subjects, the boundaries of muscles could be easily identified, while they tended to become ambiguous after repeated MAB sessions. At rest, there were involuntary worm-like movements of a specific muscle group observed in all patients. Contrary to our expectation, in all but one patient abnormal contraction was limited only in a part of synergists responsible for the abnormal posture. In normal subjects there was no abnormal contraction at rest, and all the synergists were simultaneously activated by the voluntary neck deviation. Normal subjects could not mimick the pattern of muscle activity in dystonic patients. The echo-guided MAB was performed in 16 patients. We could easily observe the diffusion of lidocaine and ethanol into the targeted muscle, and injected portions of the muscle stopped their activities just after MAB. The effect persisted for 3-4 days in at least 5 out of 10 patients who had follow-up examination. On the other hand, the movement stopped only temporarily after the injection of saline or lidocaine only. In 3 out of 16 patients, some of the uninjected synergists were activated as if to substitute for the treated muscle just after the injection. We conclude that cervical echomyography is useful to investigate the pattern of muscle activity in cervical dystonia and to accurately localize the contracting muscles during MAB.