The application of nuclear magnetic resonance imaging (NMRI) to the direct three-dimensional measurement of flow in rough-walled water-saturated rock fractures is presented for the first time. The study demonstrates the abilities of NMRI to noninvasively measure rock-water interfaces and water flow velocities in these fractures and investigates the effects of wall morphology on flow patterns inside a typical rock fracture. Two- and three-dimensional flow-encoded spin-echo pulse sequences were applied. The stability and reproducibility of the water flow patterns were confirmed by analyzing two-dimensional velocity images. A variety of geometrical and hydraulic features were determined from three-dimensional velocity images, including the rock-water interfaces, the fracture aperture distribution, and the critical aperture path; velocity profiles and volumetric flow rates; flow and stagnant regions; and the critical velocity path. In particular, the effects of a sharp step discontinuity of the fracture walls and the applicability of the cubic law were examined. As a result of the complex three-dimensional geometry, velocity profiles are generally parabolic but often highly asymmetric, with respect to the fracture walls. These asymmetric velocity profiles are clustered together, with significant correlations; they are not just local random phenomena. However, theoretical considerations indicate that the effects of the measured asymmetry on volumetric flow rates and hydraulic conductivities are insignificant, in that the overall flow inside rough fractures still obeys the cubic law. The features discussed in this study emphasize the strong heterogeneity and the highly three-dimensional nature of the flow patterns in natural rock fractures and consequently the need for three-dimensional flow analysis. ¿ 1999 American Geophysical Union