Two- and three-dimensional simulation studies have been done to evaluate waterflood oil recovery in a 40-acre 5-spot pattern using horizontal and vertical well systems. The three-dimensional simulation results indicate that the parameters considered, i.e. vertical permeability-horizontal permeability ratio, injection and production rates, and reservoir thickness have little effect on waterflood oil recovery for a particular water-oil mobility ratio. For both vertical and horizontal well systems, oil recovery decreases with increasing mobility ratio. It is concluded that the mobility ratio is the dominant parameter affecting waterflood oil recovery, in line with the experimental results of Craig, Geffen, and Morse1 for conventional waterfloods. Waterflood oil recovery using a horizontal well system is shown to be higher than that obtained by using a vertical well system. A correlation has been developed which expresses volumetric sweep efficiency in a hypothetical 5-spot pattern as a function of mobility ratio. This correlation would be useful for estimating waterflood oil recovery for the ranges of reservoir parameters considered.