The role of void growth in triggering local necking in biaxially stretched sheets is investigated. A recently proposed constitutive equation for porous plastic materials is employed in conjunction with the model of localized necking introduced by Marciniak and Kuczynski. An increased initial volume concentration of voids within the incipient neck plays the role of the imperfection. The predictions of this analysis are compared with corresponding predictions based on classical plasticity theory with various types of initial inhomogeneities. It is found that the porous plastic material model predicts forming limit diagrams qualitatively in accord with experimental results. However, the results also show that any microstructural inhomogeneity that gives rise to a continually decreasing rate of hardening in the neck would be expected to predict qualitatively similar forming limit diagrams. It is also found that the hypothesis of an equivalent thickness imperfection is not necessarily appropriate for high hardening materials.