Spring steel coils have been used to occlude abnormal vessels in patients with a congenital heart malformation. However, long-term adverse effects of steel spring coils remain uncertain, although some long-term results appear to be good. The purpose of this study was to evaluate the angiographic and histologic results of coil embolization experimentally using a biodegradable atelocollagen coil that we have recently developed. The spring coil was made from a 30% atelocollagen solution mixed with a contrast medium. The delivery system consisted of a 5-F endhole catheter and a 3-F modified biotome catheter as a catch and release system. Coil embolization was percutaneously attempted in 12 dogs, and only one coil was placed in each dog so that a variety of endothelial coverage could be evaluated both in complete and incomplete occlusion. At 1 wk to 5 mo after the procedure, the occluded vessels were resected and examined histologically. Postprocedural angiography showed complete occlusion in 6 of the 12 vessels and partial occlusion in 6. Follow-up angiography showed complete occlusion in 8 and incomplete occlusion in 4. Histologic examination revealed that diffuse thrombosis around the coil loops and the exchange between blood and contrast medium were detectable as early as 7 d after embolization. Atelocollagen was gradually replaced with fibrous tissue and became markedly degraded by 5 mo. Atelocollagen spring coils can be used to occlude abnormal vessel effectively and safely. The histologic reactions and the fate of the coil seem to be within tolerable limits. This experimental study supports the feasibility of a clinical trial of this coil embolization in patients with an aorticopulmonary collateral artery.