Infants born small for gestational age (SGA) have an increased risk of neurologic and intellectual dysfunction. Most of these infants catch up in growth and attain normal height, although some do not. Whether catch-up growth influences intellectual function is not known. To analyze whether intellectual and psychological performance of males in early adulthood are associated with body size at birth or by catch-up growth in height among boys, a population-based cohort was studied. This cohort included all male singletons born without congenital malformations in Sweden from 1973 to 1978 and alive at 18 y (n = 276,033). Information from the Swedish Birth Register was individually linked to the Swedish Conscript Register. Of 254,426 conscripted males, information on intellectual and psychological performance was available for 97% and 91%, respectively. Low birth weight, short birth length, small head circumference at birth, and preterm birth increased the risk of subnormal intellectual and psychological performance. Among SGA-born males, the most important predictor was the absence of catch-up growth. Being born SGA is associated with increased risk of subnormal intellectual and psychological performance. The data strongly support the view that, for males born SGA, it is an advantage to have catch-up growth in length.