This paper describes a study of the volumetric efficiency of a variable displacement axial piston pump. A theoretical model, based on the continuity equation and properly combined with a simplified analytical treatment of leakage flows, is first presented and used to characterise the stationary behaviour of a 9 piston pump for different operating conditions and different configurations. In this study attention is focused on pump volumetric efficiency dependency on overall operational conditions as supply pressure, pump speed, leakage clearance heights, swash plate angle and port plate timing. Successively, a numerical vs. experimental comparison of steady-state pump characteristics is presented; this last part of the paper has been developed with the aim of showing the validity limits of the assumptions made to build up the numerical model. Results confirm the applicability of the simplified hypotheses introduced and validates the analysis of volumetric efficiency previously performed.