The quantity and quality of parental linguistic input is essential to children’s vocabulary development. The current analysis is part of the SPRINT project, which uses a naturalistic family intervention to investigate the relationship between parent interaction style and vocabulary development in Swedish children 18 to 36 months old. Initial measurements of the children’s vocabulary at 12 months are followed by non-concurrent onset of intervention at either 18, 24 or 30 months. During each three-month intervention period, parents are given access to video material highlighting “best practice” adult-child interaction sessions and commentary on child language development. Parents also supply vocabulary measurements using the Swedish version of the CDI and audio recordings of interaction with their children before, during and after accessing the video material. Ten children from the first intervention cohort were chosen for this analysis. Five of these children scored in the lowest quartile on the vocabulary inventory reported at 18 months of age and five children had scores in the 50-65th percentile range. From the recordings of parent-child interaction, transcriptions of child-directed utterances were made for a total of 40 minutes per child. The transcriptions were quantitatively analyzed for MLU, lexical diversity, syntactic complexity and use of feedback morphemes. A comparison of the data for parents of children with low versus average vocabulary scores indicates qualitative differences in interaction styles. The vocabulary development of the two groups of children also reveals clearly different trajectories. The data suggest an interrelationship between parent interaction style and children’s vocabulary development.