After an earthquake, non-negligible residual displacements can occur if the isolation system does not possess a good restoring capability. In this paper the residual displacement of friction pendulum isolation systems is investigated through an extensive experimental campaign of controlled-displacement and seismic input experiments, both under uni-directional motion. A sequence of natural earthquakes was applied in three different lubrication conditions of sliding surfaces, corresponding to High, Medium and Low Friction. In order to improve current design method of sliding isolation systems, a simple equation for the estimation of the maximum expected residual displacements is proposed and compared with the experimental results.