The effects of blade deformation under running conditions on the performance of a highly loaded transonic mixed flow impeller were investigated. Two impellers were manufactured, one using the “running” blade profiles as designed and one using the converted “unrunning” or “cold” geometry. Both impellers were tested experimentally and investigated numerically. The test data taken with smooth casing showed that at maximum speed, the isentropic efficiency and pressure ratio of the running geometry was higher than the unrunning geometry by about 0.4% and 1.4%, respectively. However, the difference in performance diminished in the presence of recirculating casing treatment. Numerical calculations suggested that the differences at high speeds were mainly due to the variation in the impeller tip clearance. The calculations using deformed blade profiles under centrifugal load only, predicted performance differences which were about twice as high as the measured values. However, closer predictions were obtained when the effects of pressure loads on blade deformation were included using closely coupled fluid-structural analyses.