This work deals with modelling of flow in a complete mixed-flow pump with volute, including the tip leakage flows, using both the quasi-steady and true transient models of rotor/stator interaction. The CFX-TASCflow CFD package from AEA Technology is applied to calculate flows for a wide range of flow rates from about 0.2 to 1.4 Qopt. Fairly detailed flow structures have been predicted based on the flow rates, especially the impeller inlet recirculation and separations on the suction side of the blades for the suboptimal rates of flow, as well as strong secondary flows and separations in the volute for off-design conditions. The rotor/stator interaction influence on flow phenomena both in the impeller and volute have been investigated with very interesting results, providing a good insight into the dynamics of flow close to the volute tongue. Based on the computational results, pump performance curves (Q-H, Q-nh and Q-P, nh being the hydraulic efficiency) have been obtained. The data from this numerical investigation have been used to improve the inlet part of the impeller blades, especially close to the tip. The geometry modifications have resulted in reducing cavitation in the impeller as well as the noise at suboptimal rates of flow.