We studied five patients who received local injections of botulinum toxin for dystonic disorders to determine if there had been any distant effects on neuromuscular transmission. No patient developed weakness or abnormalities on routine electrophysiologic testing. In all patients who received more than 245 U of toxin, SFEMG in the extensor digitorum communis muscle, a muscle distant to all those injected, was abnormal. Fiber density was increased in the two patients who received the largest cumulative dose. Jitter was maximally increased at slow firing rates, confirming its presynaptic basis. Our results reveal that there is an effect on neuromuscular transmission in muscles distant to those injected with botulinum toxin for dystonia.