Effects of breakfast size on blood glucose, mood, short-term memory and concentration were assessed in 319 adolescents (age 13-20 years) in real-life setting. Mean energy increase of 63% over habitual breakfast had no effect on blood glucose or late morning mood. High energy intake from breakfast had a beneficial effect on immediate recall in short-term memory evaluated on the whole sample. However, concentration appeared to be impaired by a high caloric breakfast. There were no differences in the studied parameters according to energy supplement size. The present results are consistent with suggestions that meal size supplement has an effect on cognitive function.