Dilation is the most common inherent "failure mode" of knitted Dacron aortic prostheses. However, the incidence of graft failure related to dilation is unknown because of insufficient data regarding the occurrence, degree, and prognostic significance of postoperative alterations in graft size. In 1979 we reported that postoperative ultrasonic examinations in 95 asymptomatic patients, selected at random after aortic surgery, revealed dilation of the aortic portion of knitted Dacron bifurcation grafts. The average follow-up was 33 months, and the mean dilation was 18%. This article provides the results of a later follow-up, averaging 175 months, in 32 patients from the 1979 study. CT scanning, rather than ultrasonic imaging, was used to evaluate all parts of the grafts. CT scans disclosed that the three parts of each graft dilated, but the parts did not always dilate uniformly. Mean percent dilation values for the entire series were aortic portion 67% +/- 38% SD; right limb 77% +/- 66% SD; left limb 54% +/- 26% SD. Maximum percent dilation for any part of the grafts ranged from 26% to 367% with a mean of 94%. At least one part of the grafts dilated 100% or greater in 12 patients. Three patients had generalized and saccular dilation of the aortic portion of the prostheses and an anastomotic aortic aneurysm; one patient had a localized aneurysm in the midportion of the right limb. Three of the latter grafts were replaced with woven Dacron prostheses.