Objective: To investigate whether there is an association between catch-up growth and cognitive performance in humans. Study design: Catch-up growth was defined as the change in weight standard deviation scores during the first 2 years of life. Cognitive performance was assessed with psychometric IQ tests, administered at ages 12 and 18 years. Data were collected in twin pairs, and analyses were carried out within pairs. Results: There was a significant negative association between catch-up growth and IQ at both ages 12 and 18 years. Conclusions: A larger gain in weight during the first 2 years of life is associated with a lower IQ. However, catch-up growth is correlated with birth weight and this correlation may explain part of the association. © 2009 Mosby, Inc. All rights reserved.