The results of extensive laser measurements carried out in the blade passages of a newly designed backswept impeller are presented and discussed. Noticeable distortions of the throughflow patterns and a distinct swirling flow character were found inside the rotor. The measurement results and a simple theoretical approach suggest that the distorted throughflow patterns and the secondary flows are caused by a vortex flow. Although the relative flow has been significantly decelerated a comparatively smooth velocity profile has been identified at the rotor discharge that differed widely from the well-known jet/wake-type flow pattern.