In connection with the Hungarian radioactive waste disposal program a detailed study of the mass properties of the potential host rock (granite) has been carried out. Using the results of this study the various parameters (orientation, length, intensity, transmissivity, etc.) describing a fracture set were estimated on the basis of statistical considerations. These estimates served as basic input parameters for stochastic hydrogeological modelling of discrete fracture networks (DFN), which is a strongly developing area of hydrology, providing geologically realistic geometry for site investigations. The synthetic fracture systems generated were tested against some (but not all) field observations. The models built up on the basis of the statistical descriptions showed the same equivalent hydraulic conductivity for the modelled region as the field measurements. In addition, the models reproduce the observed hydraulic head-scattering along vertical boreholes. On the basis of the stochastic simulations of the fracture system some input parameters for the performance assessment of the planned repository were investigated. Calculation of flows into a planned disposal tunnel indicated that if the hydraulic conductivity of the material in the tunnel is the only variable parameter then there are two thresholds: under 1x10^–9 m/s and above 1x10^–5 m/s further change of the hydraulic conductivity does not dramatically affect the inflow.