A 65-year-old woman was referred to us for treatment of an aneurysm, found incidentally by abdominal ultrasonography. Angiography demonstrated a saccular aneurysm of the gastroduodenal artery and absence of blood flow from the celiac axis. The blood flow in the hepatic artery, splenic artery, and other arteries originating from the celiac axis was supplied by the superior mesenteric artery through one dilated and elongated pancreaticoduodenal artery and the gastroduodenal artery. The aneurysm was resected, and the inflow and outflow arteries were reconstructed with end-to-end anastomoses. Pathologic examination of the aneurysm sac showed diffuse intimal thickening with focal atheromas. We speculate that the increased blood flow compensating for the absence of blood flow from the celiac axis was an etiologic factor predisposing to the formation of this gastroduodenal artery aneurysm.