Permanent selective occlusion by detachable balloons with parent vessel preservation is considered to be the endovascular treatment of choice of intracranial aneurysms. It has been proposed that replacement of contrast material within the balloon with a polymerizing substance will eliminate balloon deflation. Despite this solution, our clinical experience with polyisoprene rubber balloons shows that deflation can still occur when the balloons are filled with poly(2-hydroxyethyl methacrylate). Experimental data are presented that demonstrate the chemical incompatibility between hydroxyethyl methacrylate (HEMA) and polyisoprene rubber. The resulting degradation of the polyisoprene rubber accounts well for balloon failure. Thein vitro behaviour of silicone balloons and of a new HEMA sponge formulation to fill the balloons are compared.