Increased glucose metabolism detected by (18)FDG PET imaging in unsuspected extra-temporal vessels locations of Horton's disease. Clinical observations and review of literature. Purpose. - We report three cases of Horton's disease, in which F18-Fluorine-2-Deoxy-D-Glucose ((18)FDG) positron emission tomography (PET) demonstrated a clinically unsuspected extra-cranial vessels hypermetabolism. Methods. - Fully corrected whole-body PET was performed in three patients (two women, one man) for exploring a marked inflammatory syndrome. Scanning was acquired 60 min after i.v. injection of 222 MBq of (18)FDG in average. Results. - In two patients with histologically proven Horton's disease, PET alone showed increased glucose metabolism involving the carotid and sub-clavian arteries as well as the ascending aorta, aortic arch, thoracic and abdominal aorta, and the iliac and femoral arteries. In the third patient, by detecting cervical, thoracic and abdominal vessel hypermetabolism, PET non-invasively contributed to the diagnosis of giant cell arteritis. All patients had complete clinical and biological response to corticoids. PET controls performed 3- to 6-months post-treatment, confirmed the disappearance of the metabolic stigma. Conclusion. - (18)FDG PET may show an increased glucose metabolism in asymptomatic extra-cranial vessels locations of Horton's arteritis. If these observations are confirmed on controlled trials, PET could be particularly useful for non-invasive diagnosing, staging and monitoring atypical clinical forms of Horton's disease. The metabolic imaging could also contribute to a better understanding of the pathogenesis of GCA. (C) 2002 Editions scientifiques et medicales Elsevier SAS.