High-energy linear accelerators are increasingly used in the medical field. However, the unwanted photo-neutrons can also be contributed to the dose delivered to the patients during their treatments. In this study, neutron fluxes were measured in a solid water phantom placed at the isocenter 1-m distance from the head of an 18-MV linac using the foil activation method. The produced activities were measured with a calibrated well-type Ge detector. From the measured fluxes, the total neutron fluence was found to be (1.17 +/- 0.06) x 10(7) n/cm(2) per Gy at the phantom surface in a 20 x 20 cm(2) X-ray field size. The maximum photo-neutron dose was measured to be 0.67 +/- 0.04 mSv/Gy at d(max) - 5 cm depth in the phantom at isocenter. The present results are compared with those obtained for different field sizes of 10 x 10cm(2), 15 x 15cm(2), and 20 x 20cm(2) from 10-, 15-, and 18-MV linacs. Additionally, ambient neutron dose equivalents were determined at different locations in the room and they were found to be negligibly low. The results indicate that the photo-neutron dose at the patient position is not a negligible fraction of the therapeutic photon dose. Thus, there is a need for reduction of the contaminated neutron dose by taking some additional measures, for instance, neutron absorbing-protective materials might be used as aprons during the treatment. Copyright (C) 2015, Published by Elsevier Korea LLC on behalf of Korean Nuclear Society.