Retrospectively gated phase-contrast (PC) magnetic resonance velocity and volume flow measurements were evaluated in both in vitro and in vivo experiments. The accuracy of these measurements was found to be affected by the interpolation window width required in the reconstruction of retrospectively gated data. Interpolation modified the frequency content of the series of temporal measurements by decreasing the response at higher frequencies. With a series of sinusoidal flow waveforms, the frequency response of one specific implementation of retrospectively gated PC velocity measurements was experimentally determined. The experimental response agreed with the theoretical response predicted from an analysis of the interpolating function (2.2% root-mean-square difference). In vitro experiments with a simulated carotid flow waveform demonstrated errors in the systolic measurements that were a direct result of the modified frequency response. A volunteer study was also undertaken and confirmed the in vitro findings.