The resistance of the relaxed ankle to slow displacement over the joint movement range was measured on both sides of a group of hemiparetic stroke patients, in whom spasticity had been established for at least one year and who showed no clinical signs of contractures. The ankle joints of the age-matched normal subjects were flexible over most of the movement range, showing dramatically increasing stiffness only when the foot was dorsiflexed beyond 70 degrees, with a neutral range between 90-100 degrees, and a less dramatic increase in stiffness during plantarflexion. Hemiparetic patients showed identical curves to the normal subjects on the "healthy" side, ipsilateral to the causative cerebral lesion, but were significantly stiffer in dorsiflexion on the contralateral side, without change in the minimum stiffness range or during plantarflexion. Therefore significant changes in passive biomechanical properties occur at the affected ankle of hemiparetic subjects, predominantly as the result of a loss of compliance in the Achilles tendon, although an increase in the passive stiffness of the triceps surae may also occur. The contribution of these changes to the locomotor disability of hemiparetic patients is discussed.