Of 20 young students recruited at a college for nutrition, 10 followed a diet for a period of 3 weeks and 10 were assigned to a control condition. One subject in the control group dropped out. The diet consisted of approximately 70 g protein and 25 g carbohydrate at breakfast and 10 g protein and 100 g carbohydrate at dinner. In between, a protein luncheon was consumed. On the last day of the 3-week period repeated measurements of mood and performance were made, i.e. after breakfast and after dinner. Consequently, the same measurements were made 2 months later to serve as covariates in the analyses to control for base-line differences of the two groups. It was predicted that, in the morning, the performance and the vigour of the diet group would improved whereas, in the evening, performance would become worse and subjects would feel more sleepy. However, the diet group was found to have a higher anger score in the morning (probably due to the unattractive nature of the diet) and a tendency to have a higher fatigue score in the evening compared with the controls. In addition, the diet group performed better in the morning on finger tapping, compared with the control group. With respect to memory scanning, the diet group performed more slowly in the morning in comparison with the control group. From these conflicting results, it was concluded that dietary composition had no effect on mood and behaviour. © 1990.