Objective: To study familial and nonfamilial environmental influences on attention problems and attention-deficit/hyperactivity disorder (ADHD) in monozygotic twins discordant and concordant-high and low for these traits. Method: Ninety-five twin pairs from The Netherlands Twin Register were selected. Longitudinal survey data were collected at 1, 2, 3, 5, 7, 10, and 12 years from parents, twins, and teachers. Mothers participated in a structured clinical interview when twins were between 10 and 17 years of age. Results: Affected twins from discordant pairs scored higher than unaffected cotwins on multiple measures of attention problems, ADHD, and other behavior problems according to mother, teacher, and self. Behavioral discordance was evident at age 2 and all subsequent measurements. Compared with unaffected cotwins, affected twins had lower birth weight and delayed physical growth and motor development. Differences between discordant and concordant groups were reported for maternal smoking, sleeping in different rooms, and living with only one parent. Conclusions: Significant markers of ADHD are found in infancy and include low birth weight and delayed motor development. As the knowledge of specific genetic and environmental influences on ADHD increases, future studies may focus on their complex interplay. (Contains 1 table and 4 figures.)