There is a major concern in the steam-assisted gravity drainage (SAGD) process that the existence of thief zones such as top water and/or a gas cap overlying the oil sand deposit has a detrimental effect on the oil recovery. The objective of this numerical study is to investigate the mechanisms that govern the oil and steam loss during the SAGD process in the presence of a top water zone or a gas cap. The reservoir model, STARS, developed by the Computer Modelling Group (CMG) Ltd. was first validated based on history matching of two 3D SAGD laboratory experiments with a top water zone and a gas cap, respectively. The experiments that were designed to mimic SAGD processes in the Athabasca region were conducted at field conditions of reservoir pressure and steam injection temperature. It was found that STARS was capable of history matching the top water and gas cap experiments. Therefore, it is believed that the numerical simulation captured the major mechanism of oil movement from the oil zone into the top thief zones as observed in the experiments. Lab-scale numerical sensitivity study indicated that oil movement from the oil zone into the top thief zones occurred when a very small pressure gradient existed between the oil zone and the top thief zone (e.g., < 10 kPa/m). Higher-pressure gradients resulted in more oil and steam movements into the top thief zone, less oil production, and a higher steam-oil ratio.