Hydraulic machines are faced with increasingly severe performance requirements. The need to design smaller and more powerful machines rotating at higher speeds in order to provide increasing efficiencies, has to face a major limitation: cavitation. A two-dimensional numerical approach, by means of Computational Fluid Dynamics (CFD), has been developed for studying the effect of cavitation in the volumetric efficiency of external gear pumps. Several cavitation models and grid deformation algorithms have been studied, and a method for simulating the contact between solid boundaries has been developed. The velocity field in the inlet chamber has also been experimentally measured by means of Time-Resolved Particle Image Velocimetry (TRPIV) and results have been compared to the numerical ones in order to validate the accuracy of the model. Our two-dimensional model is not able to predict the real volumetric efficiency of the pump, since several simplifications are involved in it. Nevertheless, this model shows to be valid to understand the complex flow patterns that take place inside the pump and to study the influence of cavitation on volumetric efficiency. The influence of the rotational speed of the pump has been analyzed, as well as the effect of the geometry of the inlet chamber, the working pressure, the inlet pressure loss factor, and the flow leakage through the radial clearances of the pump between gears and casing.