Since 2003 children in England have been formally assessed at the age of 5 after their first year in school, and their numerical scores reported to parents and analysed at school and national levels. The use of statutory assessment for this age group is unique in the UK, where other regions use less formal methods of assessment. It is also unusual internationally. This paper examines the peculiarity of this assessment system, the Early Years Foundation Stage Profile, using data from two ethnographic case studies of classrooms of four- and five-year-old children in London. The study revealed tensions between the construction of teachers' knowledge, their ambivalence in relation to the numerical data they report, and the use of the data for school accountability purposes. Alternative methods of assessing this age group in other parts of the UK are used to consider the implications of the production of numerical assessment data in early childhood education.