Three-dimensional (3-D), time-dependent Eulerian-Lagrangian simulations of the turbulent gas-solid flow in cyclone separators have been performed. The gas flow is simulated with the lattice-Boltzmann method. It solves the filtered Navier-Stokes equations, where the Smagorinsky subgrid-scale model has been used to represent the effect of the filtered scales. Through this large-eddy representation of the gas flow, solid particles with different sizes are tracked. By viewing the individual particles (of which there are some 107 inside the cyclone at any moment in time) as clusters of particles (parcels), the effect of particle-to-gas coupling on the gas-flow and particle behavior at modest mass-loadings (up to 0.2 kg dust per kg air) is studied. The numerical approach is able to capture the effect of mass loading on the swirl intensity as reported in the experimental literature. The performance of a Stairmand high-efficiency cyclone under various loading conditions is systematically studied. The presence of solid particles causes the cyclone to lose swirl intensity. Furthermore, the turbulence of the gas flow gets strongly damped. These two effects have significant consequences for the performance of the cyclone. The pressure drop monotonically decreases with mass loading. The collection efficiency responds in a more complicated manner to the mass loading, with mostly increased cut sizes, and increased overall efficiencies. (c) 2008 American Institute of Chemical Engineers.