Purpose: This research explored whether sustained maternal responsivity (a parent-child interaction style characterized by warmth, nurturance, and stability as well as specific behaviors, such as contingent positive responses to child initiations) was a significant variable predicting vocabulary development of children with fragile X syndrome through age 9 years. Method: Fifty-five mother-child dyads were followed longitudinally when children were between 2 and 10 years of age. Measures of maternal responsivity and child vocabulary were obtained at regular intervals starting at age 2.9 years. Sustained responsivity was indicated by the average responsivity measured over Observations 2-5. Responsivity at the 1st time period, autism symptoms, and cognitive development were used as control variables. Results: After controlling for development and autism symptoms, the authors found significant effects for sustained responsivity on receptive vocabulary, expressive vocabulary, and the rate of different words children produced through age 9. Conclusions: Maternal responsivity, which is typically a variable of interest during early childhood, continues to be a significant variable, predicting vocabulary development through the middle childhood period. Thus, responsivity is a potential target for language interventions through this age period.