In the last decade both the number of powerful CT scanners and the number of patient investigations increased with a consequent increase of the patient radiation dose. There are large dose variations (more than 10 fold) among individual CT centers and it remains true if they include the same model of CT scanners. The aim of this study was to compare the patient effective dose (De) during the CT examinations from different manufacturer models presuming the same acquisition protocol and using the imPACT patient organ and effective dose calculator program. The following manufacturers and models were selected in the program: GE (LightSpeed Ultra, LightSpeed 16, LightSpeed Pro 16, LightSpeed VCT), Philips (Brilliance 16, Brilliance 16 Power, Big Bore, Brilliance 64/40) and the Siemens (Emotion 6, Sensation 16, Sensation 16 Straton, Sensation 64, Definition AS). Head and body scans were defined for all cases altering tube voltages, while dose calculations were normalized to 100 mAs X-ray exposure. It was found that in body investigations the calculated D e for the GE scans were higher by 20% comparing to Siemens and Philips related values. For the head examinations the differences were larger by almost 55%. We also demonstrated that a power relationship can be found between the De and the tube voltage with the exponent of 3.02±0.14 kV-1 and 2.68±0.14 kV-1 for the body and the head scans respectively. It can be concluded that the patient effective dose could be different in CT examinations using distinct CT scanners but defining the same acquisition protocol. Considering the patient dose, this also means that the scan protocols cannot be transferred between different models without specific investigation.