The goal of this twin study was to explore the effects of perinatal neurobiological risk on 2 dimensions of early developmental outcome: mental and motor. The hypothesis was that the performance of the twin considered to be at higher risk for hypoxic insult would fall short of that exhibited by the lower risk co-twin. Twin discordance was established on 2 indirect indices of perinatal hypoxic risk: the Apgar score and the degree of need for neonatal respiratory oxygen support. The early outcome measures, the Bayley Scales of Infant Development (BSID), mental (MDI) and psychomotar (PDT) components, were obtained for 76 infants. Fifty-six infants were from 28 pairs of twins who were determined to be discordant, and 20 infants from 10 additional pairs who were found to be nondiscordant, on the risk indices. Although the average differences between the discordant co-twins in Apgar scores and in need for oxygen supplementation were small, the twins who were at higher risk for hypoxic insult had a significantly lower MDI compared to their lower risk co-twins. This within-pair effect was somewhat greater for premature, same-sex twins. No intrapair differences associated with risk level were found on the PDI. We conclude that early development of mental skills appears more sensitive to perinatal hypoxic risk than gross motor development when both outcome domains are denned according to the BSID.Complications associated with pregnancy and delivery, most notably prematurity, have been linked to increased risk for subsequent neurocognitive and neurobehavioral aberrations.These include not only developmental disorders such as autism