Fractal-based methods have been used in surface characterization with increasing success over the past years. These methods have been employed to characterize isotropic surfaces but, as yet, little quantitative consideration has been given to the characterization of anisotropic surfaces. In this work, the Hurst orientation transform (HOT) is used to characterize the surface anisotropy and directionality. The calculation of the HOT involves searching all pairs of pixels in a circular region to build a table of maximum differences. From this table, the Hurst coefficients are calculated in many directions and plotted as a function of orientation to reveal surface anisotropy. In this work, two new surface texture parameters, i.e. texture aspect ratio and texture direction, obtained from rose plots of the Hurst coefficients were used in the characterization of surface anisotropy and directionality. Applications of the HOT method to stereo and interferometric images of wear particles and X-ray images of healthy and osteoarthritic joints are also demonstrated. (C) 2000 Elsevier Science Ltd. All rights reserved.