PURPOSE: To evaluate changes of residual aneurysms according to the size of aneurysmal neck andthrombogenicity of a tungsten coil after incomplete embolization of experimental lateral aneurysms. MATERIALS AND METHODS: Eleven experimental lateral aneurysms with different aneurysmal neck size were created in the commoncarotid arteries of mongrel dogs. They were then divided into narrow-neck(n=3), wide-neck(n=6) and spontaneouslythrombosed control(n=2) groups. After confirmation of aneurysmal patency, incomplete embolizations of varyingdegrees (about 30% to near total occlusion) were performed using 5mm-diameter tungsten coils. Angiography wasperformed immediately before and after, and one and six weeks after embolizations. The size of residual aneurysmwas measured on each angiogram. After the last angiography, embolized aneurysms were excised and examined underlight and electron microscopes. RESULTS: On angiograms obtained 6 weeks after embolization, all residual narrowneck aneurysms were completely occluded, whereas in those with a wide-neck, therre was either no change (n=4) or aslight increase in size(n=2). On light microscopy, all narrow-neck aneurysms showed total organized fibrosis whileall control aneurysms and half those with a wide neck showed unorganized thrombi. The embolized group showed ahigher degree of organization in the aneurysmal cavity than did the control group. Neointima formation was seen inall embolized aneurysms, but no aneurysm showed foreign body reaction. On electron microscopy, uniform thicknessof plasma coatings was noted on the surface of the tungsten coils. CONCLUSION: A wide-neck residual aneurysm maypersist or increase in size, while one with a narrow-neck can be thrombosed after incomplete embolization withtungsten coils in a lateral aneurym. Careful consideration might be necessary in the embolization of wide-neckaneurysms. With plasma coatings on its surface and organized fibrosis, tungsten coil can be an useful forembolization of an aneurysm.