OBJECTIVE Postoperative dilation of gelatin-sealed knitted dacron prostheses (Gelseal, Vascutek, Scotland UK) was studied. METHODS Prospective study. METHODS Institutional clinical research. METHODS Ten patients who underwent aortic repair for abdominal aortic aneurysm (AAA) with the Gelseal graft were studied (Group A). Prior to and immediately following the implantation, the diameter of Gelseal was measured with slide calipers. The external diameter was examined by CT scan 4 weeks following the surgery. Another 10 patients, who had undergone AAA repair with the Gelseal graft more than 2 years earlier (Group B), also were examined by CT scan. Early and late dilation of Gelseal was estimated and the data from the two groups were compared. METHODS In Group A, the preimplantation external diameter was 22.6 +/- 2.3 mm while the mean package size (PS) used was 18.7 +/- 2.1 mm prior to implantation. Just after the implantation, the external diameter changed to 22.7 +/- 2.4 mm. Four weeks later, it averaged 24.0 +/- 2.2 mm. In Group B, the PS averaged 19.2 +/- 2.0 mm, while the external diameter measured by CT scan averaged 27.8 +/- 3.4 mm at a mean of 31.3 months following the surgery. RESULTS Prior to implantation, the external diameter was 1.21 times that of the PS. Following implantation, it was 1.21 times (immediately after), 1.28 times (4 weeks later), and 1.45 times (31.3 months later) of the PS. CONCLUSIONS At an average of 31.3 months following implantation, the external diameter was 45% larger than the PS. Considering that the external diameter was 21% larger than the PS even prior to implantation, the actual dilation rate is estimated as 24% of the PS in the late postoperative period.