Fourteen pairs of monozygous twins who differed from one another mainly in terms of intrauterine growth the one being greater than or equal to 25% lighter at birth than the other were studied at a mean age of 13 years. There had been no major problems during pregnancy or the neonatal period and no serious diseases during childhood. The twins grew up in the same environment. At follow-up height, head circumference, global IQ and performance IQ were reduced in the originally light twins. Intrauterine growth retardation of a certain magnitude thus has a long-term deleterious effect upon growth and development.