Numerical modeling of electromigration stress buildup and flux divergence was undertaken. The objective is to provide a mechanistic understanding of the question: Do preexisting stress voids grow during later electromigration, to become potentially fatal? Conflicting experimental results have been reported. In this work the stress field in aluminum interconnects containing voids was first quantified using the finite element analysis. The averaged stress field then served as the initial condition in the one-dimensional partial differential equation of electromigration stress buildup. The finite difference method was employed to solve the evolving stress profile and the associated atomic flux and flux divergence along the conducting line. It was found that a large preexisting void suffers greater flux divergence and is therefore more prone to growth during electromigration. A single large stress void is more detrimental than populous small voids. Published experimental observations seem to support these findings.