The purpose of the investigation was to examine the developmental trajectories of bilingual preschoolers' comprehension of Spanish and English and to determine whether a lengthy summer vacation impacted children's development during the preschool years. Participants included 83 bilingual children who were followed over a 2-year period during which time children attended a federally funded preschool programme for children from low-income homes living in the US. Children were divided into two groups based on whether their scores on receptive language measures increased or decreased during their first year of Head Start. Results revealed that children whose scores increased experienced positive growth in their language comprehension in Spanish and English over the 2-year period, whereas children whose scores decreased during the first year continued to experience a negative developmental trajectory in their second year. Additionally, it was found that a lengthy summer vacation had a differential effect on children's development. Summer vacation had a negative effect on the developmental trajectories of children who experienced gains in their comprehension of English and Spanish and a positive impact on children whose scores declined during the school year. Clinical implications suggest that children may require differential support during the school year and summer vacation depending upon their developmental trajectories during the first year in preschool. (Contains 2 figures, 11 tables, and 1 note.)