An ankle-foot orthosis (AFO) for hemiplegic patients called the DACS (dorsiflexion assist controlled by spring) AFO was developed. Previous studies have shown the DACS AFO to have the following desirable characteristics: 1) the magnitude of the dorsiflexion assist moment and the initial ankle angle of the AFO can be changed easily, and 2) no plantarflexion assist moment is generated. DACS AFOs were used daily by 5 hemiplegic patients, and their evaluations were favorable. In this paper, the structure and the mechanical characteristics of the DACS AFO and the gait improvement of hemiplegic patients with the DACS AFO are shown.