Objective: Ankle foot orthoses (AFOs) are used by nearly 50% of children with cerebral palsy (CP) to ameliorate gait impairments. The methods used to prescribe and tune the mechanical properties of an AFO, including its angular stiffness about the ankle, are based on the intuition and experience of the practitioner. The long-term goal of this research is to develop and deploy a technology-based solution to prescribing passive AFOs that uses an AFO emulator to be used in the clinic that can, under computer control, vary its stiffness in real-time to determine the best stiffness for walking. The objective of this project was to design and bench-test a first-generation wearable hydraulic ankle exoskeleton, and to conduct a small clinical trial to determine whether walking in a conventional plastic AFO was the same as walking in the hydraulic exoskeleton whose stiffness was programmed to match that of the conventional AFO. Methods: The hydraulic ankle exoskeleton was comprised of a wearable ankle exoskeleton tethered by small-diameter hydraulic hose to a push-behind cart that contained the hydraulic power supply and control components. The ankle component contained a novel double-ended cylinder with a cable anchored to the piston. The system was controlled to emulate a rotary spring. Bench top tests were performed to validate the performance of the system. In addition, an early feasibility clinical trial was conducted with five children with cerebral palsy who walked in three conventional AFOs (flexible, medium and stiff) and the hydraulic AFO controlled to match each stiffness. Kinematics and dynamics of gait were measured with a 12-camera motion capture system and a force plate. Results: The weight of the wearable exoskeleton plus shoe was 1.5 kg, 60% over the design goal. The system, running at a rail pressure of 141 bar (2,050 psi), could produce 62 Nm of torque and could emulate springs from 1 to 4.6 Nm/deg, the stiffness range of most conventional AFOs. Once calibrated, the torque-displacement properties were similar to the matched conventional AFO. Walking metrics were the same for hydraulic and conventional AFOs. Interpretation: Small-scale hydraulics are effective for a wearable exoskeleton that is designed to mimic a passive AFO and hydraulics can be used to emulate a rotary stiffness. While heavier than the design target, the added weight of the hydraulic system did not seem to impact walking in a significant way. The metrics used to evaluate walking were not sensitive enough to detect any subtle differences between walking with the hydraulic system and walking in a normal AFO.