Giant cell arteritis (GCA) and polymyalgia rheumatica (PMR) are two common rheumatic diseases occurring in middle-aged and older persons. Their cause is unknown and in neither is there a single specific diagnostic test. As a result a combination of findings is needed for their diagnosis. The American College of Rheumatology has established criteria for the classification of GCA using two methods. These criteria are best used in research studies involving patients with a diagnosis of vasculitis. One method is based on the so-called traditional format. In this method the patient with vasculitis is classified as GCA if he/she manifests any 3 among the list of 5 criteria selected. The second method, the tree format or recursive partitioning method, starts with the clinical finding that best separates patients with GCA from others with vasculitis and then uses other criteria successively to point to a final decision regarding the presence or absence of GCA. Diagnostic criteria for GCA have not been formulated. Diagnostic criteria have been established for PMR by analysis of a series of patients, but in practice most rheumatologists use criteria established informally by consensus.