We sought to determine whether the response to varying doses of botulinum toxin type A (BTX-A) injected in BTX-A-treated torticollis patients differed from the same injections given in toxin-naive individuals. We have developed a technique to objectively measure muscle weakness resulting from BTX injections in humans and have validated the technique in those not previously treated with BTX. We now examine BTX-A-treated torticollis patients to see if their response to BTX-A injection is similar to that of toxin-naive individuals. We injected 11 torticollis patients who had been receiving BTX-A injections with a standard 5-mouse unit (mu) dose into one extensor digitorum brevis (EDB) muscle and a varying dose into the other EDB, measuring muscle paralysis 2 weeks after the injection. Nine of the 11 patients were clinical and electrophysiologic responders. Two patients were non-responders. In the 9 responding patients the dose response curve to increasing doses of BTX-A was very similar to that seen in toxin-naive individuals. The mean muscle paralysis from the standard 5 mu dose was also similar to that previously reported in toxin-naive individuals. Torticollis patients who continue to respond clinically to BTX-A injections demonstrate essentially the same degree of muscle paralysis from the EDB injections as do subjects who have never been exposed to BTX-A.