The influence of the presence of a hopper section, consisting of a dust hopper and swirl attenuator, under gas-solid cyclones has been investigated by numerical simulation. Inclusion of this configuration in the simulations was found to have a significant influence on both flowpattern and separation efficiency. Moreover, predicted axial gas velocity profiles agreed better with independent experimental measurements. The cyclone separation was predicted to be better with the hopper section included than without, a physical reason is proposed. Comparison of simulated grade-efficiency curves with experimental data indicates improved agreement, in some cases substantially so.