A new ball bearing behavior model which predicts and locates power losses in a high-speed gearbox angular contact ball bearing, operating under axial and radial loads, is presented. Its new features come from the consideration of both cage action and effect of lubricant film thickness in the computation of bearing kinematics at equilibrium. Lubricant rheological properties are used in order to calculate hydrodynamic and elastohydrodynamic forces in each lubricated contact of the mechanism. This model considers cage and ball kinematics to be unknowns. These are obtained by setting motion equations for each bearing element. The computation and the location of power losses are given by the friction forces and the sliding speeds among the various bearing elements, i.e., contact between ball and inner or outer ring, ball-cage, and cage-ring. The total power loss varies strongly with the rotational shaft speed and the oil flow through the bearing; nevertheless, it is hardly sensitive to axial and radial loads. Finally, a significant role of the cage in the power losses is established.