In this study, the energy-storing capabilities of solid-ankle cushion heel (SACH) and Carbon Copy II prosthetic feet during the stance phase of gait were compared. A person with a unilateral below-knee amputation served as a component of the instrumentation to test the feet under dynamic loads. Ten trials per foot of bilateral stride at "free" velocity were collected with a video-based, three-dimensional data-acquisition system and two force plates. There were no differences between the prosthetic conditions in step length, single-limb support time, and swing period (analysis of variance) or in double-limb support time, cadence, and velocity (Student's t test). Angular kinematics and moments of the hip and knee were unaffected bilaterally by the type of foot. The progression of the center of pressure under the Carbon Copy II was delayed from 15% to 80% of stance as compared with the SACH foot. The Carbon Copy II showed slower unloading in late stance and a later peak propulsive force than did the SACH foot. The Carbon Copy II performed greater work in both the energy-storage (Carbon Copy II = 2.33 J, SACH = 1.16 J) and energy-return (Carbon Copy II = 1.33 J, SACH = 0.34 J) phases of stance and returned energy with 57% efficiency. Although the energy returned by the Carbon Copy II was clinically insignificant during level walking, these results confirm that it performs as an energy-storing device.