Background. Accurate and reproducible radiological assessment of shoulder replacement prostheses over time is important for identifying failure or to provide reassurance. A number of clearly defined radiological parameters have been described to help standardise the radiological assessment of prostheses. To our knowledge, this is the first study conducted to test the reproducibility and reliability of these measurements. Aim. The aim of this work was to test intraobserver reproducibility and interobserver reliability in the measurement of humeral component orientation (HCO), humeral head offset (HHO), humeral head size (HHS), humeral head height (HHH), and acromiohumeral distance (AHD.). Materials and methods. A cohort of 67 patients who had previously undergone shoulder replacement was identified. Two independent reviewers studied the same AP radiograph of each patient on two occasions, at an interval of one month. Results. There was strong agreement for measurements of humeral head size (ICC=0.83), moderate agreement for humeral head offset (0.66), humeral head height (0.68) and acromio-humeral distance (0.66) and fair agreement for humeral component orientation (0.44). Conclusions. Interobserver reliability and intraobserver reproducibility of radiological measurements are important factors to consider when designing longitudinal or multi-centre studies of shoulder replacement prostheses