Ten patients with focal dystonia of the hand, all of whom had benefited in an open-label study of botulinum toxin, were treated with botulinum toxin-A in a double-blind study. Response was assessed by three measures: (a) subjective rating, provided by patients' reports of the effect of the injections on the dystonia; (b) objective testing, consisting of manual muscle testing (MRC scale) to measure muscle strength in all patients, timing of a writing sample and counting the number of errors of writing off-the-line in six patients with writer's cramp, counting the number of errors on a standard test of transcription in two patients with stenographer's cramp, and rating by professional musicians of the performances of two patients with musician's cramp; and (c) physicians'rating, provided by a review of the patients' videotaped performance by neurologists who were unaware of which treatment was administered. Eight of the 10 patients had greater subjective improvement with botulinum toxin than with placebo, and this impression was verified by at least one objective test in six patients. Two patients failed to have a better response to botulinum toxin than to placebo, and their reports were verified by the objective tests. This study confirms the efficacy of botulinum toxin in many patients with focal hand dystonia.