Electromyographic (EMG) single motor unit potentials (MUPs) of the sternomastoid muscles (STM) were made before and after repeated treatment with botulinum type A toxin (Bx) for cervical dystonia. Post-treatment examinations were 6-25 weeks after the latest injection, when symptoms and EMG interference pattern had recurred and signs of denervation were scarce. Concentric needle EMG records of 200 motor unit potentials in 10 patients showed reduced durations and areas after treatment (P < 0.05). Increased polyphasia or satellite potentials were not observed. Macro-EMG records of 110 MUPs in 6 patients showed reduced amplitudes and areas in the injected STM when compared to the untreated side (P < 0.05). Fibre density was within the same range (1.0-1.2). The results indicate that the pattern of the terminal innervation is mainly restored even after repeated Bx treatments, but the number or size of active muscle fibres within the motor unit is reduced. The clinical relapse could be due to recovery of the original nerve terminals, or to nerve sprouts closely imitating the blocked terminal nerve twigs or both.