Pupils diagnosed with ADHD and pupils with ADHD symptoms tend to do less well at school than their symptom-free peers. This has been found to be particularly true for predominantly inattentive pupils. This paper aimed to establish the relative importance of inattention, hyperactivity and impulsivity to the academic progress of young children. A large dataset which held children's reading and maths attainment at the end of their first year at school, as well as teachers' ratings of ADHD-related behaviours based on the DSM-IV criteria was analysed. Inattention was strongly linked to under-attainment whilst impulsivity was positively related to attainment for similar levels of inattention. The item "Blurts out answers" on the teachers' rating scale was particularly important. When impulsivity acted as an overt sign of cognitive engagement it seemed to have a positive function. This raises questions about the inclusion of the "blurting out" item in the ADHD DSM criteria. (Contains 3 figures and 5 tables.)