The authors studied botulinum toxin type A therapy of severe biceps-triceps cocontractions after nerve regeneration following birth-related brachial plexus lesions. Six children (age, 2 to 4 years) were treated two to three times over a period of 8 to 12 months with 40 mouse units of botulinum toxin at two sites of the triceps muscle. Elbow range of motion improved from 0 to 25 to 50 deg to 0 to 25 to 100 deg (p < 0.05), and muscle force of elbow flexion increased from a mean of Medical Research Council classification 1.7 to 3.7 (p < 0.05). After a 1-year follow-up, there was no clinical recurrence.