The seismic demand generated by Nonlinear Dynamic Analyses is obtained by using ground motion records having different intensities. The uncertainty and the variability of the seismic demand are highly dependent on the variable adopted as intensity measure (IM). This generates the need to evaluate the dispersion of the demand measure in relation to different IMs. For this purpose, correlations between the maximum inter-story drift demand of nine reinforced concrete structures and a number of widely used ground motion intensity parameters are investigated. To determine the optimum parameter to be used as IM, two additional intensity parameters are analyzed, the spectral acceleration corresponding to the cracked and non-linear period of each structure, Sa(Tcrack) and Sa(T*), respectively. The results show that these two intensity measures have the best correlation with the deformation demand and their use produces a lower variability of the seismic demand.