In this paper we propose a new derived flood frequency distribution approach using the Nash instantaneous unit hydrograph (IUH) as the catchment's response model and analyze the sensitivity of flood frequency distributions to variations in the model parameters. We analyzed the effect of applying the Nash IUH to estimate peak flows in two basins in the province of Buenos Aires (Argentina). For the new flood frequency distribution, the Nash IUH parameters were estimated using the method of moments and a technique that relates the IUH parameters to the basin geomorphologic and hydraulic features. After applying the proposed derived flood frequency distributions to the basins, it is concluded that the new model (which includes the Nash IUH) yields similar results to those attained with a model developed by Raines and Valdés; the new model parameters can be estimated using a technique for ungauged basins. Also observed is a change in the shape of the flood frequency curves according to the IUH parameter estimation technique and to the set of infiltration parameters used.