Twenty preschoolers with specific language impairment (SLI) and 20 age matches with normal language (NL) participated in a study to determine whether phonological memory or semantic knowledge predicted word-learning success. Poor learners' performance was analyzed to investigate whether phonology or semantics contributed more to word-learning difficulty. Results suggest that existing lexical knowledge, as measured by the Peabody Picture Vocabulary Test (PPVT-III), and fast-mapping ability hold promise for identifying poor word learners, but individual PPV-r-III scores must be compared with SLI group scores. Poor word learners comprehended most new words and showed sufficient semantic knowledge of their referents to draw them but had difficulty producing the words. Findings indicated that both semantics and phonology contribute to word-learning difficulty, with word production presenting the biggest hindrance to Success.KEY WORDS: language disorders-children, fast mapping, language treatment-vocabulary expansion, phonological memory, semantics