Deposition of excess dietary energy occurs if energy intake exceeds expenditure. Anthropometric differences between breast-fed and formula-fed infants due to differences in energy deposition are frequently reported. It has been suggested that infant obesity may result from decreased levels of energy expenditure on physical activity. Infant diet may therefore influence energy balance by affecting behaviour and hence energy expenditure. Behavioural and motor activity of 25 formula-fed and 25 breast-fed infants were compared using activity diaries, temperament questionnaires and actometers. There were no anthropometric differences between the two groups. The breast-fed infants fed significantly more slowly than the formula-fed infants despite taking in similar milk volumes, and also fed significantly more frequently. Otherwise there were no significant differences between the two groups with respect to the daily time spent awake and content, asleep, fussy, or crying, nor with respect to gross motor activity and temperament. It is unlikely that energy balance of the two groups differed significantly on account of behavioural activity.