PURPOSE To evaluate endoscopic fenestration as a treatment option for growing aneurysm due to a type II endoleak or endotension after endovascular aneurysm repair (EVAR). METHODS Eight patients (7 men; median age 69 years, range 55-79) who underwent "successful" EVAR were diagnosed with a growing aneurysm due to a type II endoleak (n=4) or endotension (n=4). Surgical intervention consisted of endoscopic fenestration of the sac and removal of all the thrombus material, preceded by clipping of the inferior mesenteric and all lumbar arteries in cases of endoleak. Fluid samples from the fenestrated aneurysm sac were analyzed for the presence of microorganisms and fibrin degradation products (FDP) and/or D-dimers. RESULTS The median duration of operation was 220 minutes (range 111-333). There was no perioperative mortality. In one patient, the endoscopic procedure was converted to an open fenestration procedure. Seven patients had uncomplicated follow-up and a clear decrease in the diameter of the sac; one patient was converted to open repair owing to continued sac growth despite fenestration. Bacterial cultures were negative in all patients, but high levels of FDP and/or D-dimers were found in all available samples, indicating continued fibrinolysis. CONCLUSION Endoscopic fenestration, with or without endoscopic clipping of all side branches, seems to be an effective, reliable and minimally invasive treatment option for patients with a growing aneurysm due to type II endoleak or endotension. The high levels of FDP and/or D-dimers in the aneurysm sac are suggestive of hyperfibrinolysis, which may play an important role in aneurysm growth after EVAR.