Five patients with knitted Dacron aortic bifurcation prostheses developed prosthetic dilation and anastomotic false aneurysms. The interval between placement of prostheses and secondary presentation ranged from 4 to 10 years. Three patients presented with femoral false aneurysms (one ruptured) and two presented with aortic false aneurysms (one ruptured). In all cases, anastomotic sutures remained intact on the prosthesis and there was no evidence of infection. The mean (+/- SEM) increase in diameter of prostheses was 85.7 +/- 19.1% (range 76% to 137%). Dilated prostheses were removed in four cases and were tested for tensile strength and studied with scanning electron microscopy. These were compared to normal, nondilated knitted Dacron prosthetic fabric from the same manufacturer. There was no loss of breaking strength in dilated prostheses. Furthermore, on examination with scanning electron microscopy, there was no evidence of degeneration or fracturing of individual Dacron filaments. However, dilated prostheses were found to have a greatly expanded knit. The distance between loops was significantly increased and the number of fibers per 1000 microns was significantly less in comparison to normal knitted Dacron fabric. These studies confirm an association between prosthetic dilation and false aneurysm formation. Further, they suggest that the cause of prosthetic dilation is loss of compactness of the knit not associated with intrinsic Dacron fiber deterioration. A fabrication defect is most likely responsible.