The pilot, copilot, and two additional crewmembers were preparing to depart on a night medical transport flight. The copilot was at the controls of the airplane while the pilot monitored the airplane?s systems and prepared for takeoff. During the engine start, one of the two linemen providing ground services to the airplane departed the ramp area leaving one lineman alone, which the operator allowed for normal ground servicing operations. After completing the engine start, the copilot gave the disconnect ground power hand signal to the lineman. The lineman responded with the hold hand signal and then moved to the left and rear of the airplane to disconnect the ground power unit (GPU). The pilot stated that he monitored the airplane’s voltmeter to verify that the GPU was disconnected by observing a drop in the voltage current. However, although the drop in voltage could confirm that the GPU was no longer supplying power, it could not confirm that the GPU cable was disconnected. The pilot then looked out the window and realized that the airplane was moving forward. The pilot took the controls, applied the brakes, and set the parking brake. The pilot did not have visual contact with the lineman, so he asked the two crewmembers seated in the cabin to locate him, and one of the crewmembers reported that she saw him standing next to the GPU. The pilot then began to taxi the airplane for departure within the nonmovement area of the ramp; the pilot did not realize that the GPU was still connected to the airplane. As the airplane turned right, the GPU was pulled onto its side, and it then fell onto the lineman?s right leg, which resulted in a serious injury. The pilot stopped the airplane and shut down the engines without further incident. The pilot should have ensured that the GPU was disconnected from the airplane via hand signals from the lineman providing ground services before taxiing the airplane and his failure to do so resulted in the lineman?s injury. Probable Cause: The pilot?s failure to ensure that the ground power unit was disconnected from the airplane via hand signals from the lineman providing ground services before taxiing, which resulted in an injury to the lineman.