Our aim was to ascertain the adequacy of human milk as the sole source of vitamin B6 and the associations between maternal and infant status during extended exclusive breast-feeding. Vitamin B6 status was followed in lactating mothers and their exclusively breast-fed infants by determinations of erythrocyte pyridoxal 5'-phosphate concentration and the erythrocyte aspartate transaminase stimulation test at 2 months (n = 118), 4 months (n = 118), 6 months (n = 112), 7.5 months (n = 70), 9 months (n = 36), 10 months (n = 14), 11 months (n = 11), and 12 months (n = 7) postpartum. Of the mothers, 54% had used vitamin B6 supplement during pregnancy, and all received a pyridoxine hydrochloride supplement of 1 mg/day throughout lactation. The infants had a higher vitamin B6 status than their mothers. During the first 4 months, infant vitamin B6 status was generally adequate independently of the actual vitamin status of the nursing mother. Most of the infants with low status at 2 months were those born to mothers who were not supplemented during pregnancy. By 6 months of exclusive breast-feeding, 30% of cases of low vitamin B6 status in nursing mothers were reflected in their infants. Thereafter, the risk of low vitamin B6 status in exclusively breast-fed infants increased even if the mother's status was adequate. Our findings suggest that gestationally accumulated stores are important for the maintenance of adequate vitamin B6 status of infants during the early months and that for some infants, human milk alone, without supplementary foods, may be insufficient to meet vitamin B6 needs after 6 months of age.