Ambient noise measurements may be used under certain conditions to estimate characteristics of seismic response in a given site, namely the fundamental frequency and amplification at resonance. Both parameters depend on the geological structure and the dynamic properties of soils in the study area. The method known as Horizontal to Vertical Spectral Ratio technique, HVSR (Langston, 1979; Nakamura, 1989), provides a reliable estimate of the resonant frequency in most soil sites (Borcherdt, 1970), but the associated amplification is usually underestimated (Field & Jacob, 1995; Malagnini et al., 1996; Bard, 1997; Bard et al., 1997; Riepl et al., 1998). The aim of the present study is to elaborate a method for estimating design acceleration time histories in bedrock, combining theoretical and experimental site response estimates on the basis of 1D site response and microtremor measurements. The idea consists of selecting an accelerogram at bedrock and to modify it appropriately, using an heuristic transfer function, determined from ambient noise measurements at the same site. The modified accelerogram is then used to evaluate the seismic response of the investigated site using conventional 1D linear or equivalent linear method. With this procedure it is possible to estimate relatively accurate design ground motion time histories. The methodology is applied at three sites down-town Catania (Italy), and in one site in Euroseistest (http://euroseis.civil.auth.gr) where microtremor data and seismic events are available and geotechnical characterization of their soils are well known.