Numerical experiments were carried out for yield-stress fluid flows through a 1:4 sudden axisymmetric expansion using the Herschel-Bulkley and Bingham models with a two-viscosity approximation. The influence of shear thinning, inertia and yield-stress values on the structure of the flow and on pressure and head losses was studied. The detailed structure of flow in the expansion shows an initial vortex The yield stress generates two toroidal rigid dead zones, one located in the corner of the expansion and one located close to the stagnation point that appears between the vortex and the downstream flow Inertia and yield stress act in opposite ways. When inertia increases, the vortex increases in size and the rigid dead zone decreases. Conversely, when yield stress increases, the vortex decreases in size and the rigid dead zone increases. Shear thinning reduces the dimensions of both vortices and dead zones. Pressure loss increases with yield stress. In addition, the numerical results are consistently compared with experimental ones.