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This paper presents an approach to quantify the degree of heterogeneity of a fracture network starting from the information that is collected for a geomechanical classification of rock masses. Six synthetic experiments have been used to prove the existence of a correspondence between the variability in fracture properties and in the direction and magnitude of flow. Statistical analyses conducted using fracture data collected at a tunnel crossing the north-western Italian Alps have also shown that variability in fracture characteristics is indeed related to the magnitude of inflows. These findings have proved that it is reasonable to estimate the degree of heterogeneity of a fracture network by combining the variance of those fracture characteristics that regulate both flow and the geomechanical behavior of a rock mass. An index comprising a combination of variances of fracture parameters at different scales is presented. Its dependence on scale showed that the heterogeneity of fractured rock increases with scale up to a certain scale and then gradually decreases at large scales. [Copyright &y& Elsevier]