The moderating effect of maternal warmth on the association between low birth weight and children's attention-deficit/hyperactivity disorder (ADHD) symptoms and low IQ was studied in 2,232 twins. Half of 5-year-old children had low birth weights, below 2,500 g. Maternal warmth, a component of expressed emotion, was coded from mothers' audiotaped descriptions of each child. Both parents and teachers rated children's ADHD symptoms, and the children were administered an IQ test. Results showed a significant interaction between children's birth weight and maternal warmth in predicting mothers' and teachers' ratings of ADHD. The interaction was not significant for IQ. The findings suggest that the effect of children's birth weight on their ADHD symptoms can be moderated by maternal warmth and that enhancing maternal warmth may prevent behavior problems among the increasing population of low-birth- weight children.