Atherosclerosis is a progressive vascular disease that is a major risk factor for acute myocardial infarction and stroke (Libby, 2002). An important determinant of atherosclerotic plaque vulnerability is the stability of the fibrous cap, afforded in part by the presence of collagen. The formation of intermolecular cross-links within newly formed collagen fibrils gives collagen its stability, and is essential for tissue function. Intermediate (difunctional) cross-links undergo spontaneous maturation, characterised by the production of trifunctional pyridinium compounds, pyridinoline (Pyd) (Robins & Duncan, 1983) and deoxypyridinoline (Dpd) (Ogawa et al., 1982). Therefore, the level of mature collagen cross-links within the region of the fibrous cap may dictate the vulnerability of the plaque to rupture. Plaques were obtained from patients undergoing carotid endarterectomy (CEA). CEA samples were dissected out into healthy marginal tissue, plaque tissue and underlying media for biochemical analysis of collagen cross-links by HPLC. Portions of healthy marginal tissue and whole plaque were also used for histological determination of total collagen to cellular ratio by staining with Masson’s trichrome. Detection of CD68 by IHC was used as a macrophage marker in plaque sections, and an arbitrary scoring system assigned. Total collagen and cross-link data is expressed as mean ± s.e.m and assessed for statistical significance using one-way ANOVA with Tukey’s post-hoc test. The level of Pyd cross-links (n=12) contained within the plaque was significantly higher than that found in the underlying media (p≤0.01) and healthy tissue (p≤0.01). However, levels of Dpd (n=8) were considerably lower in the plaque than in the underlying media (p≤0.01). The ratio of total collagen to cellular material (n=16) was high in plaque and underlying media (1.3±0.04), and equal in healthy tissue (1.0±0.05). The highest incidence of macrophage frequency was found to be predominant in the sub-cap region of the underlying media (n=9). These results show for the first time that it is possible to measure mature collagen cross-links in human atherosclerotic artery samples, and that Dpd in the plaque is considerably lower than that measured in the underlying media. Therefore, levels of Dpd may be crucial to the stability of the plaque, serving as a marker of vulnerability. Future work will assess the levels of bifunctional cross-links prior to maturation to pyridinium compounds, which may account for reduced Dpd.