Iron deficiency anemia has long been thought to have effects on the central nervous system (CNS). Finding direct evidence of this in human infants, however, has been challenging. Auditory brainstem responses (ABRs) provide a noninvasive means of examining an aspect of the CNS that is rapidly maturing during the age period when iron deficiency is most common. ABRs represent the progressive activation of the auditory pathway from the acoustic nerve (wave I) to the lateral lemniscus (wave V). The central conduction time (CCT, or wave I-V interpeak latency) is considered an index of CNS development because myelination of nerve fibers and maturation of synaptic relays lead to an exponential reduction in the CCT from birth to 24 mo. In 55 otherwise healthy, 6-mo-old Chilean infants (29 with iron deficiency anemia and 26 nonanemic control infants), the CCT was longer in those who had been anemic at 6 mo, with differences becoming more pronounced at 12- and 18-mo follow-ups despite effective iron therapy. The pattern of results--differences in latencies but not amplitudes, more effects on the late ABR components (waves III and V), and longer CCTs (as an overall measure of nerve conduction velocity)--suggested altered myelination as a promising explanation, consistent with recent laboratory work documenting iron's essential role in myelin formation and maintenance. This study shows that iron deficiency anemia in 6-mo-old infants is associated with adverse effects on at least one aspect of CNS development and suggests the fruitfulness of studying other processes that are rapidly myelinating during the first 2 y of life.