A general expression has been derived for the NMR signal from a fluid in steady but turbulent flow in a magnetic field gradient. The precise dependences of the spin echo amplitude on both flow and experimental parameters are described in terms of the temporal autocorrelation function for velocity fluctuations in the fluid. The correlation function width determines whether a diffusion model of turbulent eddies is valid, and for physiological flow in large vessels in NMR imaging a more appropriate measure is the turbulent intensity of velocity fluctuations. The theoretical predictions have been verified in experiments using gradient echo images of flowing water. The theory also predicts that even echo rephasing phenomena may still occur in MR imaging of turbulent flows.