Objective To examine the effect on adult cognitive function of being born small for gestational age (SGA), and to evaluate whether cognitive function is related to intrauterine growth restriction (IUGR). Study design Fifty-nine SGA subjects (birth weight <10th percentile) and 81 controls (birth weight ≥10th percentile) born at term underwent cognitive assessment with the Wechsler Adult Intelligence Scale–Third Edition at age 19-20 years. Repeated ultrasound measures of fetal growth were available for weeks 25, 33, and 37 in a subgroup of 29 SGA subjects and 75 control subjects, and these were data used to dichotomize the 29 SGA subjects into those with IUGR and those without IUGR. IUGR was defined as growth deviating more than −2 SD from the mean value of the control group. The effect of maternal smoking during pregnancy was considered as well. Group differences were analyzed using a general linear model, controlling for sex and socioeconomic status. Results The SGA group had lower full IQ scores than the control group (mean difference, −6.3; 95% CI, −2.8 to −9.7; P = .001), including lower scores on 6 of the Wechsler Adult Intelligence Scale–Third Edition subtests. In the SGA subgroup with repeated ultrasound measures, 6 of 29 subjects (21%) had IUGR, and these subjects also had a lower IQ compared with controls (mean difference, −14.0; 95% CI: −4.8 to −23.3; P = .003). Maternal smoking during pregnancy was related to lower IQ in the control group but not in the SGA group, independent of IUGR or non-IUGR status. Conclusion IQ scores were lower in young adults born SGA compared with controls. Our analysis suggest that this outcome is related to IUGR.