Nanocomposites of polycarbonate (PC) reinforced with nanosized silica particles were prepared by a melt mixing technique in an internal mixer. Two kinds of commercial hydrophilic fumed silicas differing in their specific surface area were added in amounts up to 5% by volume, and their reinforcing action was compared to that of organically modified silica, loaded in the same amounts. Particle-matrix interactions were investigated by means of rheological and dynamic-mechanical thermal analysis, demonstrating the important role played by the organic modification in the interactions with the polymer matrix, and showing an optimal nanoparticle loading around 2 vol%. The scratch resistance of the nanocomposites obtained from hydrophilic silicas was investigated, and a remarkable enhancement in the indenter's penetration resistance was observed for all the compositions with respect to pristine PC. The same behaviour was observed for the Shore D hardness and for the impact resistance of the nanocomposites that also significantly improved with the maximum load shifting from a minimum value of 521 N for pristine PC up to values grater than 1330 N for the nanocomposites, demonstrating the activation of effective mechanisms of energy dissipation due to the presence of the nanofillers.