Most aneurysms shrink after successful endovascular repair. It has been observed, however, that some aneurysms continue to enlarge despite apparent exclusion of the sac by an endograft device. Unexplained abdominal aortic aneurysm (AAA) enlargement in these circumstances appears to be associated with high pressures inside the sac, and the phenomenon has been termed endotension. This paper reviews current theories and experimental evidence regarding the possible mechanisms of causation of endotension, and early experiences with treatment.