To evaluate the effect of vitamin B6 status on infant growth, we studied longitudinally anthropometry and the erythrocyte parameters that reflect long-term vitamin B6 status [erythrocyte pyridoxal 5'-phosphate concentration (EPLP), erythrocyte aspartate transaminase basal activity (EAST0), and its activation co-efficient (alpha EAST)] in 44 infants. The infants were exclusively breast-fed for 6 mo, given additional solids according a uniform schedule from 6-9 mo, and formula after 9 mo, if needed. In seven of these infants, a low vitamin B6 status (EPLP < 10th, and EAST0 > 10th or alpha EAST > 90th percentile for these values in reference infants) was observed between 4 and 6 mo of age. These seven infants showed slower length velocity (0.30 +/- 0.05 versus 0.40 +/- 0.02 mm/d, p < or = 0.02) and deeper fall in length-for-age (-0.69 +/- 0.20 versus -0.25 +/- 0.07 SD score, p < or = 0.03) from 6 to 9 mo of age than the similarly fed infants with higher vitamin B6 status. Preceding vitamin B6 status remained a significant explanatory factor for length velocity and change in length-for-age in addition to preceding and concomitant weight velocity, when sex, birth size, preceding length gain, and mid-parent height were taken into account. Change in weight-for-age alone explained 16% and 18% and, together with vitamin B6 status, 23 and 27% of the variation in length velocity and in change in length-for-age, respectively. Thus, in healthy breast-fed infants, according to our results, low vitamin B6 status is associated with reversibly reduced gain in length.