['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.', 'To demonstrate the value of assay of "aminoterminal propeptide of type III procollagen" (PIIINP) to predict the growth of ascending TAA.', 'Thirty consecutive patients with asymptomatic ascending TAA were recruited and PIIINP was measured. The patients were then followed up in the usual way, by annual imaging tests. The patients were then divided into two groups: Group 1=patients with a stable TAA or with a slow growth rate (≤2 mm/year) (n=23) and Group 2=patients with a fast-growing TAA (>2 mm/year) (n=7). Mean age of the patients was 65.8±10.8 years. At inclusion, the mean value for maximum aneurysm diameter was 44.6±3.4 mm. The mean follow-up period was 1.6±0.8 years. The patients presenting a significant increase in maximum aneurysm size have a significantly higher PIIINP level than those demonstrating no or limited growth in maximum aneurysm diameter (4.51±1.09 vs 3.38±0.96 µg/l respectively, p=0.02). There is no difference between the two groups for the other clinical or laboratory parameters. Following multivariate analysis the PIIINP level remains significantly higher in Group 1 (p=0.03).'] Is aminoterminal propeptide of type III procollagen ( PIIINP ) associated with ascending aortic aneurysm growth rate?