The large and rapidly expanding body of literature on bilingual acquisition is mostly comprised of either single-case or cross-sectional studies. While these studies have made major contributions to our understanding of bilingual children's language development, they do not allow researchers to compare and contrast results with regard to individual differences over time. This paper aims to investigate the issue of individual differences with a longitudinal group study of 13 French-English bilingual children. The main focus is lexical development. We will examine how extralinguistic factors such as gender, parental input and birth order impact on the lexical development of the children. Using quantitative (parental checklists, questionnaires) and qualitative measures (interactions with parents), we demonstrate that language exposure and parental input are closely linked to vocabulary size, amount of language mixing and cross-linguistic synonyms. The findings call for more longitudinal group studies of bilingual acquisition in order to obtain comparable results on larger populations.