Endovascular stenting appears to be an appealing treatment modality to selected complex intracranial aneurysms. However, stents currently used for endovascular treatment are not specifically designed for the cerebrovasculature. Stent parameters, such as porosity and filament size, have to be carefully optimized for long-term successful treatment. We investigated the influence of the stent filament size on the intraaneurysmal flow dynamics in a sidewall aneurysm model in vitro. Three helical stents with 76% porosity but different filament sizes of 178, 153, and 127 microm were studied using particle image velocimetry. Twenty-four pulsatile flow conditions were investigated. The results show that stenting significantly reduces intra-aneurysmal vorticity and the mean circulation inside the aneurysm is reduced to less than 3% of its value before stenting. For constant porosity, a further reduction of the mean circulation, up to 30% can be obtained by reducing the filament diameter. For a constant Womersley number, this further reduction is accentuated with increase in the peak Reynolds number. Further reduction in the mean circulation inside the aneurysm was not achieved for the 127 microm stent. With further reduction in filament diameter, the helical stent filaments positioned against the aneurysm neck started wavering with the flow transferring added momentum into the aneurysm. For stents of smaller filament diameter, a supporting ultrastructure is required.