Previously asymptomatic aneurysms in ten patients ruptured within 36 days (mean, ten days) of a prior laparotomy. The laparotomy and associated intra-abdominal disease may have precipitated rupture of the unresected abdominal aneurysms by reduction of the collagen content of the aneurysm wall, thus making the wall weaker. The scar-like collagen fibers of an aneurysm wall provide the strength that permits the wall to resist rupture. There is a dynamic equilibrium between synthesis and lysis of this collagen. Lysis of collagen is enhanced by injury, such as laparotomy, and by nutritional depletion and local inflammation. Collagen lysis is greatest in the area adjacent to the injury, but also occurs at remote sites as well. Lysis is greatest during the first postoperative week, after which, in the absence of sepsis or starvation, synthesis exceeds lysis and the equilibrium is restored. A thin aneurysm wall may be weakened enough during this period of negative collagen balance to allow rupture.