An unusual centrifugal impeller disk failure has been encountered in one of the stages of a multistage air compressor. The failure occurred when a triangular piece of metal separated from the disk at the rim of the impeller disk. Stress analyses show that the steady hoop stresses at the rim were very low. Metallurgical observations indicate that the failure was caused by high cycle fatigue in cast 17-4 PH steel. A modal analysis shows that the failure can be explained by a resonance condition. The excitation of the backward rotating tangential nodal pattern by the stationary vaned diffusers just downstream of the impeller rim causes this resonance condition. Avoiding the resonance condition by changing the number of diffuser vanes has eliminated the failures.