The relationship between breakfast composition and cognitive performance was examined in elementary school children. Two experiments compared the effects of two common U.S. breakfast foods and no breakfast on children's cognition. Using a within-participant design, once a week for 3 weeks, children consumed one of two breakfasts or no breakfast and then completed a battery of cognitive tests. The two breakfasts were instant oatmeal and ready-to-eat cereal, which were similar in energy, but differed in macronutrient composition, processing characteristics, effects on digestion and metabolism, and glycemic score. Results with 9 to 11 year-olds replicated previous findings showing that breakfast intake enhances cognitive performance, particularly on tasks requiring processing of a complex visual display. The results extend previous findings by showing differential effects of breakfast type. Boys and girls showed enhanced spatial memory and girls showed improved short-term memory after consuming oatmeal. Results with 6 to 8 year-olds also showed effects of breakfast type. Younger children had better spatial memory and better auditory attention and girls exhibited better short-term memory after consuming oatmeal. Due to compositional differences in protein and fiber content, glycemic scores, and rate of digestion, oatmeal may provide a slower and more sustained energy source and consequently result in cognitive enhancement compared to low-fiber high glycemic ready-to-eat cereal. These results have important practical implications, suggesting the importance of what children consume for breakfast before school.