The impact of breakfasts of different glycaemic loads on the performance of nineteen children, aged six to seven years, was explored. Over a four week period, children attended a school breakfast club each day and ate one of three meals. Each meal offered a similar amount of energy but differed in their glycaemic load. When working individually, the behaviour of a child was rated in the classroom every ten seconds for 30 min to produce a measure of time spent on task. Memory was assessed by asking for the recall of a series of objects. The ability to sustain attention was measured by asking for a response after various delays. The incidence of negative behaviour was recorded when playing a video game that was too difficult to allow success. Two to three hours after a low glycaemic load breakfast had been consumed, performance on the tests of memory and the ability to sustain attention were better, fewer signs of frustration were displayed and initially more time was spent on task when working individually in class. The importance of the results was discussed in the context of the wide range of factors that influence behaviour in school.