The purpose of this study was to examine early fast mapping abilities in late talkers (LT) and typically developing (TD) Spanish-speaking children by considering the effect of different variables on fast mapping (age, vocabulary level, grammatical category and number morphology). Thirty-eight Spanish-speaking children were assessed at three times (21 to 24 months at the beginning of the study). A group of LT was matched in age with a TD control group. They completed a fast mapping task with a disambiguation phase and an extension phase. Results show that vocabulary level and age interact with grammatical category and number morphology. TD children's performance was significantly higher than LT children's, but these differences decreased with age. Results suggest that the incorporation of new labels does not work automatically; some children may need repeated exposures to word--referent pairings. Longitudinal results suggest the importance of looking at the dynamics of lexical acquisition in addition to vocabulary size.