Small amyloid deposits commonly occur along the internal elastic lamina of the temporal artery. In temporal artery biopsies from 22 patients with histological signs of giant cell arteritis and 25 without, amyloid deposits were found in 14 and 21 biopsies, respectively. Two specific peptide antisera show that this amyloid is identical to the recently identified medin-amyloid in the ageing aorta. On immunoelectron microscopy, the amyloid appeared topographically closely related to the elastic material. Furthermore, fragmented elastic material was often immunolabelled for medin and found to be engulfed by giant cells. Medin is an internal fragment of the larger precursor lactadherin and is presumably formed by specific enzymatic cleavage events. In situ hybridization showed that lactadherin is expressed locally by smooth muscle cells of the temporal artery. Given the potential role of lactadherin as a mediator for the adhesion of cells, including macrophages, to other cells or surfaces, lactadherin or its fragment medin may be important in the inflammatory process in giant cell arteritis.