The material balance (MB) equation for stress-sensitive undersaturated and saturated naturally fractured reservoirs (nfr's) has been written taking into account the presence of an unsteady-state naturally fractured aquifer (nfa). The nfa can be of limited or infinite size. The solution is presented in finite difference form to achieve a quick convergence of the iteration process. Historically, non-fractured aquifers have been attached to material balance calculations of naturally fractured reservoirs. This is not realistic from a geologic point of view as it implies that fractures are present in the oil portion of the reservoir, but disappear the moment the water oil contact is reached. It is shown that the use of an unfractured aquifer in a naturally fractured reservoir can lead to erroneous oil recovery estimates. The effect of water influx on the material balance equation is illustrated with an example.