In Tuscany (Central Italy) hillslopes are often affected by landslides that mainly involve the shallow regolith (i.e. the first 2 m) and the soil. An assessment of the factors controlling the geotechnical and hydrological features is crucial to understand the occurrence of slope instability mechanisms. The aim of this study is to improve the reliability of the outputs of the deterministic HIRESSS (HIgh REsolution Slope Stability Simulator) model. This software makes use of Monte Carlo simulation techniques to manage the uncertainty inherent in the geotechnical and hydrological input parameters. In situ determinations of geotechnical properties were made to assess the variation and uncertainty in these parameters. The data collected allowed a map of the input parameters for HIRESSS to be made, and a case study in the northern-western part of Tuscany have been evaluated. So far, the difficulty in determining the soil initial saturation state didn’t allowed to achieve a fully satisfying result in terms of prediction reliability. However, a step further has been made and the simulations performed making use of the in situ collected data are expected to show substantial improvement in the results compared to the use of literature parameters.