The possibility that the taking of vitamin supplements may influence cognitive functioning was explored. One hundred and twenty-seven young healthy adults took either ten times the recommended daily dose of nine vitamins, or a placebo, under a double-blind procedure, for a year. After 12 months better performance on two measures of attention was found in females who had taken the vitamin supplement, even though the blood status of nine vitamins reached a plateau after 3 months. The use of regression equations demonstrated the association between improved thiamin status and improved performance on a range of measures of cognitive functioning in females rather than males. Although it was not possible to establish the reason for a beneficial response in females rather than males, the evidence that females respond differently to dietary factors was discussed.