Plasma levels of PIIINP a marker of extracellular matrix metabolism activity have been linked to abdominal aortic aneurysm growth rate, but its relationship with thoracic aortic aneurysm (TAA) growth has never previously been studied. The PIIINP molecule is an indicator of both the synthesis and the degradation of type III collagen. Occasionally, the removal of PIIINP from the newly-synthesized type III collagen is not complete and the propeptide may remain attached to some of the molecules, known as type IIIpN-collagen. PIIINP is a degradation product of type IIIpN collagen. PIIINP has a molecular weight of 42 000 and contains three distinct domains: a triple-helical domain (Col 3) in the middle of the molecule, the Col 1-domain at the aminoterminal and Col 2-domain at the carboxyterminal end of the propeptide. PIIINP is cleared from the circulation by scavenger receptors in liver endothelial cells.