Nuclear magnetic resonance imaging is applied to measure flow patterns in natural, water-saturated, rough-walled rock fractures. From three-dimensional water density and velocity vector images the fracture morphology and flow patterns are determined. The parabolic nature and asymmetry of the velocity profiles, and thus the accuracy of local cubic law flow rate predictions, vary greatly. This depends on the degree of wall roughness. Particularly complex flow patterns are found in one sample which contains a sharp fracture wall discontinuity. A power law for the flow rate versus aperture for the low-flow region was found without considering the hydraulic gradients. ¿ 1999 American Geophysical Union