The paper presents the ATP-EMTP implementation of a Monte Carlo procedure aimed at evaluating the backflashover rate (BFOR) of an HV overhead line (OHL). The ATP-EMTP circuit model of the OHL includes detailed line insulation and lightning representation; spatially extended and/or involved grounding systems are represented by a new, simplified model which reproduces the effects of propagation and soil ionization phenomena; statistical input data concerning lightning polarity, lightning stroke parameters (peak current, front and tail times), lightning location, line insulation and phase angle of the supply voltage. An external software engine generates all the required statistically-oriented ATP-EMTP input data, sequentially launches and manages ATP simulations and post-processes all results. An application on a 150 kV - 50 Hz typical Italian OHL is reported and discussed.