We have investigated the dosimetric properties of a commercial kilovoltage cone beam computerised tomography (kV-CBCT) system. The kV-CBCT doses were measured in 16 and 32 cm diameter standard cylindrical Perspex computerised tomography (CT) and Rando anthropomorphic phantoms using 125 kVp and 1.0-2.0 mA s per projection. We also measured skin doses using thermoluminescence dosimeters placed on the skin surfaces of prostate cancer patients undergoing kV-kV image matching for daily set-up. The skin doses from kV-kV image matching of prostate cancer patients on the anterior and lateral skin surfaces ranged from 0.03 +/- 0.01 to 0.64 +/- 0.01 cGy depending on the beam filtration and technique factors employed. The mean doses on the Rando phantom ranged from 3.0 +/- 0.1 to 5.1 +/- 0.3 cGy for full-fan scans and from 3.8 +/- 0.1 to 6.6 +/- 0.2 cGy for half-fan scans using 125 kVp and 2 mA s per projection. The isocentre cone beam dose index (CBDI) in the 16 and 32 cm Perspex phantoms is 4.65 and 1.81 cGy, respectively (using a 0.6 cm(3) Capintec PR06C Farmer chamber) for full-fan scans, and the corresponding normalised CBDIs are 0.72 and 0.28 cGy/100 mA s, respectively. The mean weighted CBDIs are 4.93 and 2.14 cGy, and the normalised weighted CBDIs are 0.76 and 0.33 cGy/100 mA s for the 16 and 32 cm phantoms, respectively (full-fan scans). The normalised weighted CBDI for the half-fan scan is 0.41 cGy/100 mA s for the 32 cm diameter phantom. All measurements of the CBDI using the 0.6 cm(3) Farmer chamber are within 2-5% of measurements taken with the 100 mm CT chamber. The CBDI technique and definitions can be used to benchmark CBCT systems and to provide estimates of imaging doses to patients undergoing on-board imager (OBI)/CBCT image guided radiation therapy.