Children start school with differing levels of skills. Thus, children of different social origin have different probabilities of educational success right from the start of their school career. This paper analyses how the gap in language abilities of children with different social backgrounds develops from age three to five. A focus lies on the question whether pre-school education can help to close this gap. The data of the UK Millennium Cohort Study (MCS) show that children's score on a standardized vocabulary test strongly depends on their parents' education. These social differences remain stable or even increase slightly over the two-year period. Using fixed effect models, it is demonstrated that children of higher educated parents can improve their vocabulary more strongly than children whose parents have a lower educational level. Participation in an early education institution positively affects the vocabulary development of children with lower educated parents while there is no significant pre-school effect for children of higher educated parents. The results indicate that pre-school attendance does not lead to a catching-up process of children with lower educated parents. But without pre-school attendance, the gap between children of higher and lower educated parents widens even further.