Type III collagen contributes to the tensile strength of the aortic wall, and mutations in the type III collagen gene have been suggested as the basis for the familial tendency to abdominal aortic aneurysm (AAA). Variation in this gene was investigated in 153 patients with AAA, 87 with aortoiliac stenosis and 98 age-matched controls. The rare mutation at amino acid 619 of Gly [RIGHTWARDS ARROW] Arg, previously associated with AAA in a single family, was not found in any of the patients with aneurysm. For the Ala [RIGHTWARDS ARROW] Thr variation at amino acid 531, the frequency of the threonine allele was 0·25 in patients with AAA and stenosis, compared with 0·35 in controls. The frequency of the rare allele in the region 3' to the gene demonstrated by Ava II digestion (0·27 in the general population) was found to be 0·29 in the AAA group and 0·19 in the stenosis group (P = 0·023). In the AAA group the presence of the Ava II rare allele was associated with a significant increase in aneurysm diameter (P = 0·044). Non-invasive assessment of aortic distensibility was available in 25 patients: those carrying the Ava II rare allele had less distensible aortas than those not carrying this allele (median pressure-strain elastic modulus 42·0 and 23·9 N/cm2 respectively, P = 0·008). Variation in the type III collagen gene may influence the mechanical properties of the ageing aorta and hence its susceptibility to disease and dilatation. In contrast, there is no evidence for a single common founder mutation in type III collagen predisposing to AAA.