Abstract
In this paper I present some of the results of a three-year case study of a mathematics department in a UK school that taught in setted' groups. Interviews, observations, questionnaires, and assessment data are used to show the way in which high ability students, particularly girls, underachieved and became disaffected as a result of being in the top set'. Various qualitative and quantitative results of the case study are used to show the way in which top sets can diminish, rather than enhance, achievement for high ability students. It is also suggested that the gender inequalities in mathematics achievement that continue to prevail among the top five per cent of students may partly be caused by features of top set' learning.