The hereditary spastic mouse was studied as a model of cerebral palsy in childhood to test the hypothesis that intramuscular botulinum toxin A would prevent the development of calf-muscle contractures. A prospective randomised controlled trial of calf injection with botulinum A compared with injection of normal saline was performed on juvenile mice. At maturity, the calf muscles of the spastic mice were 16 per cent shorter than those of their normal siblings. The calf muscles of spastic mice injected with botulinum toxin A grew to within 2 per cent of normal length. This difference in mature muscle length was highly significant.