Objective: The aims of this study were to estimate the effective radiation doses from CT examinations of both adults and children in Japan and to study the impact of various scan parameters on the effective doses. Methods: A questionnaire, which contained detailed questions on the CT scan parameters employed, was distributed to 3000 facilities throughout Japan. For each scanner protocol, the effective doses for head (non-helical and helical), chest and upper abdomen acquisitions were estimated using ImPACT CT Patient Dosimetry Calculator software v. 1.0.4 (St George's Hospital, London, UK). Results: The mean effective doses for chest and abdominal examinations using 80-110kV were significantly lower than those using 120kV. However, there was no statistically significant difference in the mean effective doses for head scans between facilities employing 80-110kV and 120kV. In chest and abdominal examinations, the mean effective doses using CT scanners from Western manufacturers [Siemens (Forchheim, Germany), Philips (Eindhoven, Netherlands) and GE Medical Systems (Milwaukee, WI)] were significantly lower than those of examinations using Japanese scanners [Hitachi (Kashiwa, Japan) and Toshiba (Otawara, Tochigi, Japan)], except for in paediatric chest examinations. Conclusion: The mean effective doses for adult head, chest and abdominal CT examinations were 2.9, 7.7 and 10.0mSv, respectively, whereas the corresponDing mean effective doses for paediatric examinations were 2.6, 7.1 and 7.7mSv, respectively. Advances in knowledge: Facilities using CT scanners by Western manufacturers commonly adopt low-tubevoltage techniques, and low-tube-voltage CT may be useful for reducing the radiation doses to the patients, particularly for the body region.