Height and weight measurements for a large sample of twins from birth to nine years were used to construct growth curves for this period. Compared to singletons, twins were substantially smaller at birth, particularly for weight (30% deficit), but the latter showed a dramatic recovery in the first 3 months, then smaller increments until finally reaching the singleton norms at 8 years. Height, while less depressed at birth (17% deficit), was also slower to recover during the first year, then moved in comparable increments with weight until reaching parity with singletons. Prenatal influences on birth size were largely dissipated as each twin converged on his/her genetic growth curve. Monozygotic twins became progressively more concordant with age (R = 0.94 for height), while dizygotic twins became less concordant (R = 0.49). Ten monozygotic pairs with large birth-weight differences (greater than 750 g) were examined at 6 years; the smaller twin was still significantly lighter (P less than 0.01), marginally shorter (P less than 0.10), but not significantly lower in IQ. The results suggested a high degree of buffering for intelligence against the effects of nutritional deficit in the prenatal period. Some factors in addition to placental anastomosis are suggested as a possible basis for long-term size differences in monozygotic pairs.