A method for determining flow by nuclear magnetic resonance (NMR) imaging is described. A conventional spin-echo imaging sequence is employed with the addition of balanced gradient pulses on either side of the pi radiofrequency pulse. Flow velocities in the direction orthogonal to the image plane are determined by the phase shifts in the NMR image. Experimental validation of the technique in vitro was achieved with a phantom designed to give a continuous flow of water. The flow rate measured by NMR agreed well with the volume flow rate through the phantom. In vivo, NMR flow measurements on the carotid and femoral arteries of two volunteers were compared with Doppler ultrasound results. Velocity measurements were in general agreement. Rapid changes in flow are difficult to follow with the NMR method unless particular care is taken in gradient profile design. The technique can readily be used in existing NMR imaging machines and may have a useful clinical role.