This article describes a longitudinal analysis of a nationally representative cohort of over 80,000 pupils in England who completed both national end of Key Stage 2 (KS2) tests and the Cognitive Abilities Test (CAT) at age 11 in 1997, national end of Key Stage 3 (KS3) tests at age 14 in summer 2000 and General Certificate of Secondary Education (GCSE) and other public examinations at age 16 in summer 2002. The CAT had significantly higher correlations with subsequent KS3 and GCSE outcomes than did KS2 test points scores. However, multiple regression analyses indicated that a combination of CAT and KS2 test scores gave the best prediction of future KS3\GCSE outcomes. The article argues that measures of both pupils' general transferable learning abilities, and measures of specific curricular attainments at the end of primary school have unique and distinct value at the start of the secondary phase. The article discusses some practical ways in which the different types of assessment data can be used within the secondary school. (Contains 1 figure, 1 note, and 5 tables.)