Computational fluid dynamics (CFD) techniques have been refined for modeling the hemodynamics in cerebral aneurysms. Recent interest has focused on understanding hemodynamic changes by treatment with a flow diverter (FD), i.e. a stent with a dense metal mesh which is placed across the ostium to divert the majority of flow away from the aneurysm. Potential complications include remnant inflow jets but, more seriously, aneurysm hemorrhage. For optimization of treatment outcome, a better understanding of the effects caused by the FD would be beneficial. In particular, pressure and velocity distributions at the aneurysm ostium are of interest, as they will be directly affected by the FD which in turn will influence post-treatment hemodynamics inside the aneurysm. Here, we report the results of a CFD study investigating the relationship between pre-treatment and post-treatment velocities, pressures and wall shear stresses (WSS) in the aneurysm with corresponding hemodynamic conditions at the aneurysm ostium prior to treatment. The study was carried out using a dedicated CFD prototype which allows modeling the effects of a virtual FD integrated into patient-specific geometries utilizing Darcy's law. Velocities and WSS were reduced in all cases post FD treatment, pressure increased in one case. Heterogeneous distributions of the velocity magnitude were found at the ostium with focal maxima indicating potential risk zones for remnant inflow jets into the aneurysms. Pressures at the ostium correlated with pressure changes inside the aneurysm which could become a pre-treatment indicator for the evaluation of the suitability of a particular aneurysm for FD treatment.