Initial setup of a Cardio Pulmonary Resuscitation device (100) is to be accomplished so that a contact pad (102) of the device makes the correct contact with the chest before the device starts giving chest compressions to the patient. Thereby situations where the contact pad is not in initial contact with the chest or situations where the contact pad initially compresses the chest too much are avoided. The position of the contact pad is measured during initial setup, and when contact between the chest and the contact pad is established, the actual position of the contact pad is measured and set as an initial position. Then a positioning range is set relative to the initial position. Therefore, the CPR device is provided with a positioning aid (131) which informs the rescuer when the contact pad is within the positioning range or when the contact pad is outside the initial positioning range.