ABSTRACT. The objective of the study was to evaluate an association between low birth weight (LBW) and attention-deficit hyperactivity disorder (ADHD) attending to potential family-genetic and environmental confounders. We examined 252 ADHD cases (boys and girls) and 231 non-ADHD controls and their parents. All subjects were extensively assessed with structured diagnostic interviews, cognitive assessments, and structured interviews of prenatal, infancy, and delivery complications. ADHD cases were three times more likely to have been born LBW than were non-ADHD controls, after attending to potential confounders such as prenatal exposure to alcohol and cigarettes, parental ADHD, social class, and comorbid disruptive behavior disorders in parents and offspring. If this association was causal, 13.8% of all ADHD cases could be attributed to LBW. These results converge with prior studies documenting similar associations and indicate that LBW is an independent risk factor for ADHD. Children with LBW, however, make up a relatively small proportion of children with ADHD.