Experimental aneurysms with a high propensity to spontaneous rupture were used to assess the efficacy of endosaccular packing with platinum coils. Lateral aneurysms, surgically constructed in pigs from vein allographs, were untreated or treated by partial or total endovascular packing with coils. All untreated aneurysms (4/4) ruptured and most (3/4) partially treated aneurysms also ruptured. None of 6 aneurysms in which the lumen was occluded by 90% or more ruptured or demonstrated regrowth on subsequent angiography. Partial treatment was therefore not effective in preventing delayed haemorrhage. These findings suggest that subtotal endovascular treatment may not be protective after aneurysmal subarachnoid haemorrhage.