It has been suggested that a low-glycaemic index (GI) breakfast may be beneficial for some elements of cognitive function (e.g. memory and attention), but the effects are not clear, especially in adolescents. Thus, the aim of the present study was to examine the effects of a low-GI breakfast, a high-GI breakfast and breakfast omission on cognitive function in adolescents. A total of fifty-two adolescents aged 12–14 years were recruited to participate in the study. Participants consumed a low-GI breakfast, a high-GI breakfast or omitted breakfast. A battery of cognitive function tests was completed 30 and 120 min following breakfast consumption and capillary blood samples were taken during the 120 min postprandial period. The findings show that there was a greater improvement in response times following a low-GI breakfast, compared with breakfast omission on the Stroop (P = 0·009) and Flanker (P = 0·041) tasks, and compared with a high-GI breakfast on the Sternberg paradigm (P = 0·013). Furthermore, accuracy on all three tests was better maintained on the low-GI trial compared with the high-GI (Stroop: P = 0·039; Sternberg: P = 0·018; Flanker: P = 0·014) and breakfast omission (Stroop: P < 0·001; Sternberg: P = 0·050; Flanker: P = 0·014) trials. Following the low-GI breakfast, participants displayed a lower glycaemic response (P < 0·001) than following the high-GI breakfast, but there was no difference in the insulinaemic response (P = 0·063) between the high- and low-GI breakfasts. Therefore, we conclude that a low-GI breakfast is most beneficial for adolescents' cognitive function, compared with a high-GI breakfast or breakfast omission.