The aim of this study was to uncover factors accounting for the ability of children with specific language impairment (SLI) to learn agreement morphemes in intervention. Twenty-five children with SLI who participated in a six-month intervention program focused on teaching third person singular -s or auxiliary "is"/"are"/"was" showed a wide range of use of the target morpheme after intervention. Regression analyses showed that age and two factors expected to be related to agreement--the use of noun plural -s and subject/verb constructions prior to intervention--significantly predicted progress in the acquisition of agreement morphemes. In contrast, the pretreatment use of morphemes hypothesized to be unrelated to agreement was not a significant predictor of progress. The results indicate that the ability of children with SLI to learn agreement morphemes relies on their prior ability to use noun plural and subject/verb constructions.