The aim of this study was to assess how the glycaemic potency (blood glucose (BG)-raising potential) of breakfast is associated with cognitive function (CF) in school children, taking into account important confounders, including iron status, underlying physiological adaptations and socio-economic status.Sixty children aged 11-14 years were selected on the basis of having breakfast habitually. Their breakfast and any snacks eaten on the morning of the study were recorded. They were categorized into four groups according to the glycaemic index (GI) and glycaemic load (GL) of the breakfast: low-GI, high-GL; high-GI, high-GL; low-GI, low-GL and high-GI, low-GL above or below the median for GI=61 and GL=27. BG levels were measured in finger-prick blood samples immediately before and immediately after the CF tests.A low-GI, high-GL breakfast was associated with better performance on a speed of information processing (P<0.01) and a serial sevens (P<0.001) task 90 min later; a high-GI breakfast with better performance on an immediate word recall task (P<0.01); and a high-GL breakfast with better performance on a Matrices task (P<0.01).GI, GL or both were associated with performance on the majority of the CF tests (4 of 7) used. This study describes the macronutrient composition of breakfast that could have a positive influence on the cognition of school children, proposes the use of both GI and GL to estimate exposure, and discusses future directions in this area of research.