Objective.To test the reliability of Outcome Measures in Rheumatology Clinical Trials (OMERACT) consensus-based ultrasound definitions for normal and vasculitic temporal and axillary arteries in patients with giant cell arteritis (GCA) and in controls.Methods.A preliminary 1-day meeting and a full 3-day meeting fulfilling OMERACT Ultrasound Group guidelines were held. Temporal and axillary arteries were examined at 2 timepoints by 12 sonographers on 4 patients with GCA and 2 controls. The aim was to test inter- and intrareader reliability for normal findings, halo sign, and compression sign. In both meetings, patients had established GCA. Pathology was more recent in the full meeting, which was preceded by 6 h of training. Scanning time was 15–20 min instead of 10–13 min.Results.In the preliminary exercise, interreader reliabilities were fair to moderate for the overall diagnosis of GCA (Light κ 0.29–0.51), and poor to fair for identifying vasculitis in the respective anatomical segments (Light κ 0.02–0.46). Intrareader reliabilities were moderate (Cohen κ 0.32–0.64). In the main exercise, interreader reliability was good to excellent (Light κ 0.76–0.86) for the overall diagnosis of GCA, and moderate to good (Light κ 0.46–0.71) for identifying vasculitis in the respective anatomical segments. Intrareader reliability was excellent for diagnosis of GCA (Cohen κ 0.91) and good (Cohen κ 0.71–0.80) for the anatomical segments.Conclusion.OMERACT-derived definitions of halo and compression signs of temporal and axillary arteries are reliable in recent-onset GCA if experienced sonographers (> 300 examinations) have 15–20 min for a standardized examination with prior training and apply > 15 MHz probes.