This study was undertaken to analyze immediate and mid-term knitted Dacron graft dilation and to establish which parameters should be taken as a reference when aortic graft dilation is evaluated. A Dacron knitted microvel double velour vascular graft (Hemashield Gold) was implanted in 30 patients with aneurysmal (19 cases, 63%) or occlusive (11 cases, 37%) aortic disease. The stems of bifurcated prostheses (27 patients, 90%) and tube grafts (3 patients, 10%) were measured. The package sizing (labelled size) was compared with the external diameter measured intraoperatively with a slide caliper, prior to implantation and after complete clamp release. Additional measurements were obtained by ultrasound 1 and 6 months after implantation, and in 16 cases (53% of the patients) ultrasound and computed tomography (CT) were performed at the end of the first year. The means of the measurements were compared using Student's t test for matched pairs. The statistical significance level was set at p values < 0.05. There was a statistically significant difference between the package sizing (15.3 +/- 1.1 mm) and the external diameter measured prior to implantation (18.7 +/- 1.3 mm); and with the external diameter following implantation (19.6 +/- 1.4 mm), (p < 0.01). External diameters measured prior to grafting and following implantation (after complete clamp release), when compared with the manufacturer's size, showed a mean increase in graft diameter of 3.4 mm (22%) and 4.3 mm (28%), respectively. There were no statistical differences between the external diameter measured after clamp release (19.6 +/- 1.4 mm) and the size determined by ultrasound 4 weeks (19.3 +/- 1.2 mm) and 6 months (19.8 +/- 1.5 mm) following surgery (p values 0.11 and 0.56, respectively). Considering size after clamp release as a reference (19.6 +/- 1.4 mm), an almost significant (p = 0.08) increase in the diameter (0.7 +/- 1.5 mm) was obtained at the end of the first year when the measurement was performed with ultrasound. However, when the measurement was performed by CT at the end of the first year, the differences (0.9 +/- 1.6 mm) revealed statistical relevance (p = 0.04). There was no statistically significant difference between the sizes obtained by ultrasound (20.3 +/- 2.1 mm) and by CT (20.5 +/- 2.2 mm) at the end of the first year (p values 0.07). The package sizing is not a reliable parameter for choosing the size of knitted Dacron grafts. Immediate increase in diameter noted in Dacron grafts is caused by discrepancies between the package sizing and the measured diameter after clamp release during implantation, and by an initial adaptation of the textile structure. This must be taken into account for an accurate investigation of the immediate graft dilation rate, and if further follow-up is contemplated, a measurement to be taken as a reference should be performed by ultrasound or CT in the immediate postoperative period.