Objective measures (kinematics and kinetics) were used to study prospectively the effects of botulinum toxin A (BTX/A) on the gastroc-soleus muscle in ambulant children with cerebral palsy. In this prospective before and after trial, 15 children with diplegia and 10 children with hemiplegia were studied (mean age 5 years 7 months, range 4 years to 9 years). A range of standardized clinical measures was undertaken but the emphasis for this report is on the three-dimensional gait analysis (3DGA) results. All children showed improvements in sagittal ankle kinematics, as has been previously reported. Two new measures of ankle kinetics were devised: ankle moment quotient (AMQ), and ankle power quotient (APQ). Before intervention, ankle moments were characterized by a 'double bump' ankle moment. A typical abnormal baseline ankle-power curve was triphasic with an initial trough of absorption followed by abnormal mid-stance power generation, instead of the usual A1 pattern, and reduced terminal stance power generation (A2). Three weeks after treatment with BTX/A alone there was a statistically significant improvement of AMQ and APQ; some patients required potentiation of BTX/A with a short period of serial casts. Both groups (BTX/A alone and BTX/A plus casting) continued to show improvement in ankle kinetics from baseline after 12 and 24 weeks. This is the first study to demonstrate improvements in the typical abnormal ankle kinetics which we believe provides evidence of the 'biomechanical transformation of muscle'.