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Because of inadequate response to or intolerable side effects of oral medication, nine patients with segmental, generalized, and focal myoclonus were treated with intramuscular botulinum toxin type A. All patients were evaluated with neuroimaging, routine and limb-monitored electroencephalography, electromyography, evoked potentials and appropriate biochemical studies. Patients were aged 2 to 22 years, with duration of myoclonus from 1 month to 10 years. Multiple medication trials included antiepileptic drugs, benzodiazepines, tryptophan, L-dopa/carbidopa, baclofen, and dantrolene. Patients were injected with botulinum toxin in their affected area with electromyographic guidance to affected muscles with different doses (8 to 20 units/kg), except two patients who were injected with 32 and 45 units/kg, respectively, at 4- to 8-month intervals. One patient did not complete botulinum toxin treatment because of subjective weakness, although there were virtually no side effects reported in patients completing therapy. Patients reported a dramatic reduction in painful myoclonus. In addition, patients exhibited improved functional skills, as demonstrated by markedly improved use of affected extremities and improvements in ambulation. One patient, who was nonambulatory prior to treatment, was able to walk afterward. Long-term benefits could be related to higher dosage used or negative feedback effect. (J Child Neurol 1999;14:781-786).