Purpose. - This work evaluated the interobserver variability in cone beam computed tomography (CBCT) registration for prostate cancers treated with intensity-modulated radiotherapy. Material and methods. - Twelve technologists realized 286 CBCT/CT registrations (bone registration followed by prostate to prostate registration). The registration results were compared to those obtained by two radiation oncologists (reference). Each technologist reported the shifts calculated by the software in all three axes. A statistical analysis allowed us to calculate the minimum threshold under which 95% of the observers found similar values. A variance analysis followed by the post hoc test were used to find differences in interobserver registration variability and determine whether any individual users performed registrations which differed significantly from those of the other users. Results. - The registration differences compared to the reference in the three directions in terms of 95th percentile are: 2.1 mm left-right, 3.5 mm target-gun, 7.3 mm anterior-posterior. In the posterior direction, 4% of the observers have found differences superior to 8 mm, margin used in routine without the use of a daily CBCT. The variance test revealed a P-value <0.05 only for target-gun and for all observers there was no significant difference compared to the reference. Conclusion. - This study confirmed the interest of a 3D tissue registration for prostate treatments. The registration study showed a good interobserver reproducibility. This showed the importance of a daily CBCT/CT registration in prostate treatment with the possibility of a planning target volume margin reduction in the three directions. An evaluation of a partial delegation of registration to technologists should be done by the radiation oncologists. (C) 2015 Societe francaise de radiotherapie oncologique (SFRO). Published by Elsevier Masson SAS. All rights reserved.