The treatment of a dissecting, relatively large aneurysm in the vertebrobasilar circulation (section P1 of arteria cerebri posterior), i.e. treatment of a risky aneurysm, is presented. On the 5th day after the treatment with a flow-diverter, a thalamic infarction occured probably due to the closure of the ipsilateral paramedian perforators of the proximal arteria cerebri posterior.