Purpose: To evaluate the effects of using nonword (NW) stimuli in treatment of children with phonological disorders relative to real words (RWs). Methods: Production data from 60 children were examined retrospectively. Thirty of the participants were previously treated on sounds in error using NWs, and the other 30 had been treated using RWs. Generalization was the dependent variable, with measurement of accurate production of treated and untreated sounds immediately posttreatment and longitudinally following the withdrawal of treatment. Results: Under both stimulus conditions, and at both sampling points in time, there was greater generalization to treated sounds compared with untreated. NWs, as opposed to RWs, induced greater, more rapid systemwide generalization as a function of treatment. Children exposed to NWs sustained those levels of performance even after treatment was withdrawn. Children exposed to RWs eventually reached comparable levels of phonological generalization, but not until 55 days after the cessation of treatment. Conclusion: The findings support the ecological validity of NWs in phonological treatment. The differential results hint that NWs may benefit treatment efficacy and efficiency, but this remains to be determined through prospective study. Consideration is given to a potential theoretical account of the NW effects, with appeal to the literature on novel word learning. (Contains 3 figures.)