Rain-induced landslides are major geotechnical hazards. The influence of infiltration under various rainfall, ground conditions on slope stability is still poorly understood. In this paper, a finite element parametric study was carried out to investigate the influence of different rainfall events and ground conditions on transient pore water distributions in unsaturated soils. A steep, unsaturated colluvium hillside in Hong Kong was chosen: the initial water table, rainfall intensity and rainfall duration were variables. Pore water pressures predicted during the transient seepage analyses were used as input groundwater conditions for subsequent limit equilibrium analyses of the stability of the slope. Shear strength variation due to the presence of matrix suction was also taken into account. It was found that the factor of safety not only depended on the intensity of rainfall and the initial groundwater table, but also on rainfall duration. A critical rainfall duration was identified, when the factor of safety was the lowest.