Summary. This prospective analysis assessed the risk of mild mental retardation (MMR) associated with low birthweight (LBW) in the Child Health and Development Studies. Scores of 50–70 on the Raven Progressive Matrices, a relatively culture-free test of cognitive functioning, were used to categorise MMR. At the age of 5,13.8% of the 195 children with birthweights less than 2500 g (LBW) were MMR, whilst 4.2% of the 2293 children with normal birthweights (>2955 grams) were MMR. After adjusting for confounders (maternal age, race, education, prenatal alcohol use, maternal conditions, and congenital anomalies), the relative risk of MMR for LBW was 3.4 (95% CI 1.2–5.4). For children aged 9–11, 7.7% of 194 LBW children were MMR, compared with 6.2% of the 2546 with normal birthweights; the adjusted relative risk for LBW was 1.2 (95% CI 0.7–2.0). Although a strong association between LBW and MMR was observed for both blacks and non-blacks at the age of 5, the association between birthweight and MMR was apparent only for blacks in the cohort of children aged 9–11. These findings suggest that race, a marker for environmental factors which were not measured in this study, may modify the LBW and MMR relationship.