Botulinum toxin type A (btx A) injections are the most effective treatment for most patients with focal dystonia. Some patients who improve after btx A injections and later lose response have serologic evidence of antibodies to btx A with the mouse neutralization assay (seropositive patients). Another group of patients who lose response to btx A do not have detectable antibodies (seronegative patients). Seropositive patients may improve after injections of botulinum toxin type F (btx F), an alternative serotype of botulinum toxin. We treated nine seronegative resistant patients with btx F. None of these patients had muscle atrophy after injection with btx A. Five of the nine had improvement after btx F injection that was sustained for at least three consecutive btx F injections. This observation is consistent with the hypothesis that btx resistance in seronegative patients is caused by undetected antibodies to btx A. If this be the case, then there may be techniques for preventing or reversing btx resistance, as in the case of resistance of factor VIII in the treatment of hemophilia A.