Twenty-nine children with cerebral palsy and triceps surae hypoextensibility were divided into 2 groups. In group I trophic regulation of the muscle was defective and in group II it was normal. Torque values were plotted against tibiocalcanean angles before and after treatment, which consisted of muscle lengthening by successive plaster casting or of surgical elongation of the tendon or the aponeurosis. In group I, casting had no effect, but surgery increased passive dorsiflexion and gave definite clinical improvement without modifying the range of passive muscle stretch. In group II, with normal muscle adaptation, plaster casts were successful when tolerated. Children in this group required braces after surgery to prevent rapid recurrence of hypoextensibility. Both casting and surgery increased the passive muscle stretch. There was a displacement of the starting point of the passive and active curves in both groups. This is an unavoidable side-effect which makes walking on tiptoes impossible after casting or surgery.