The authors investigated the role played by various types of linguistic structures in child-directed speech in early vocabulary acquisition in Brazilian Portuguese. Thirty-five Portuguese-learning children in Brazil were observed as they interacted with their mothers at 9, 13, and 18 months of age. Although a large fraction of the speech addressed to the children throughout the study consisted of sentence fragments, maternal use of single word utterances did not correlate with child vocabulary growth, either concurrently or longitudinally. In contrast, mothers’ use of certain sentence constructions (e.g., copulas and questions) predicted children’s acquisition of nouns, even after we controlled for variations in their vocabulary at the onset of the study. These results suggest that children rely on linguistic structure to learn novel words even when input is highly fragmented.