Eighteen subjects consumed low-fat/high-carbohydrate (LFHC) (29% fat, 54% carbohydrate, 15% protein), medium-fat/medium-carbohydrate (MFMC) (45% fat, 42% carbohydrate, 12% protein), and high-fat/low-carbohydrate (HFLC) (62% fat, 24% carbohydrate, 13% protein) isocaloric lunches in random order on 3 separate days. The MFMC lunch was similar in energy and macronutrient composition to the habitual lunchtime intake of the subjects. A battery of cognitive performance tasks together with mood and appetite ratings were carried out before and during the 3 h after lunch. Results showed longer reaction times following the LFHC and HFLC lunches compared to the MFMC lunch, which produced an improvement in performance on this task. In addition, subjects rated themselves as more drowsy, uncertain and muddled, and less cheerful after the LFHC and HFLC lunches compared to the MFMC lunch, and as less tense after the LFHC meal. These findings indicate that the macronutrient content of lunch can have significant acute effects on subsequent mood and performance. In particular, higher than usual proportions of fat or carbohydrate produced a relative impairment of cognitive efficiency.