Motor imagery is the mental simulation of a movement without motor output. In recent years, there has been growing interest towards the application of motor imagery-based training, or "mental practice", in stroke rehabilitation. We have developed a virtual reality prototype (the VR Mirror) to support patients in performing mental practice. The VR Mirror displays a three-dimensional simulation of the movement to be imagined, using data acquired from the healthy arm. We tested the system with nine post-stroke patients with chronic motor impairment of the upper limb. After eight weeks of training with the VR Mirror, remarkable improvement was noted in three cases, slight improvement in two cases, and no improvement in four cases. All patients showed a good acceptance of the procedure, suggesting that virtual reality technology can be successfully integrated in mental practice interventions.