This paper presents the results of an investigation into the effects of partial loading on the behaviour of steel pallet racks. A six level, six bay, 3-dimensional pallet rack frame model was built using ABAQUS and a geometrically non-linear analysis conducted. Twelve different load combinations were analysed under different boundary conditions and different side imperfections. Most combinations failed by sway buckling. Depending upon the side imperfection and the base condition critical combinations involved having completely unloaded bays or lifts adjacent to two or more fully loaded bays or lifts with reductions in capacity over a fully loaded rack of up to 40%.