Background: According to a dual-route model of written language processing, spelling of irregular words provides an index of the status of lexical spelling procedures, whereas nonword spelling provides information about non-lexical processing that relies on phoneme-grapheme conversion. Because regular words can be spelled using either route, accuracy for such words may reflect the combined function of the two routes, and may be mathematically predicted on the basis of spelling accuracy for irregular words and nonwords. Aims: The purpose of the present study was to examine the application of a dual-route prediction equation, and a related multiple regression model, to evaluate pre-post treatment spelling performance of individuals with acquired alexia/agraphia. Methods and Procedures: Eight individuals with language impairment due to left hemisphere damage received behavioral treatment to improve their written spelling. Their spelling performance was examined before and after treatment on untrained word lists with regular and irregular spellings, and pronounceable nonwords, and concurrence between predicted and observed spelling of regular words was evaluated. Outcomes and Results: The group demonstrated significantly improved spelling performance after treatment, and the prediction equation and multiple regression model both accurately predicted regular word performance on the basis of irregular word and nonword scores. In addition, the multiple regression model provided potentially useful information about the relative contribution of the lexical and non-lexical routes to spelling performace. Conclusions: The prediction equation and related multiple regression model used in this study can offer novel insight into the cognitive processes available to individuals with agraphia as well as provide a quantitative means to characterize response to treatment.