The authors investigated the association between intrauterine growth and intellectual disability (ID). The appropriateness of intrauterine growth was assessed using percentage of optimal birth weight, a measure that accounts for gestational age, maternal height, parity, and infant sex. Using population-based record linkage, singleton Caucasian and Aboriginal children born in Western Australia in 1983-1992 and alive in 2002 with ID of unknown cause (n = 2,625) were compared with children without ID (n = 217,252). The odds of ID increased with less-than-optimal intrauterine growth. In Caucasian children, after adjustment for sociodemographic factors, severe growth restriction was associated with development of mild-moderate ID among preterm births (< 37 weeks) (odds ratio (OR) = 1.71, 95% confidence interval (CI): 1.06, 2.77) and term births (>= 37 weeks) (OR = 2.42, 95% CI: 1.88, 3.12) and with severe ID (OR = 4.79, 95% CI: 2.59, 8.83) among term births. Effects were similar among Aboriginal children. Severe growth restriction (OR = 3.2, 95% CI: 1.3, 7.9) and poor head growth (OR = 3.6, 95% CI: 1.4, 9.0) were independently associated with severe ID. Infants with excess intrauterine growth were more likely to be diagnosed with ID associated with autism spectrum disorder (OR = 2.36, 95% CI: 0.93, 6.03). These findings suggest that inappropriate intrauterine growth, less than or greater than optimal birth weight, is associated with development of ID.