The purpose of the present study was to understand the reciprocal, bidirectional longitudinal relation between joint book reading and English receptive vocabulary. To address the research goals, a nationally representative sample of Head Start children, the Head Start Family and Child Experiences Survey (2003 cohort), was used for analysis. The children were aged 3-4 years at programme entry. The mothers' average age at programme entry was 39 years old. A parallel process model was utilized to examine the growth factors of joint book reading and receptive vocabulary in parallel. Three significant findings emerged: (1) initial levels of English receptive vocabulary and joint book reading positively covaried; (2) English receptive vocabulary and joint book reading were positively and reciprocally related to each other; and (3) slopes for joint book reading and English receptive vocabulary negatively covaried. Results suggest that joint book reading can support and scaffold Head Start children's English receptive vocabulary. Reciprocally, Head Start children's English receptive vocabulary appears to predict the extent to which they engage in joint book reading at home. Moreover, the frequency of joint book reading decreases as the children demonstrate higher levels of English receptive vocabulary. (author abstract)