Objective: To examine whether twin birth weight differences relate to subsequent differences in conduct problems at 3 to 4 years of age. Design: Retrospective cohort study using mothers' reports on twins' behavior, birth weight, and zygosity. A partial sample participated in laboratory visits. Setting: Jerusalem, Israel. Participants: All ethnic Jewish families identified by the Israeli Ministry of the Interior as having twins born in 2004 and 2005 were sent mail surveys regarding children's development. Families of twins (n=1319) answered the survey when the twins were 3 years old. Same-sex birth weight-discordant twin pairs (n=112) were selected for the main analyses of the study. Main Exposure: A birth weight difference of 20.0% or more between the twins. Main Outcome Measures: Mothers' reports on twins' conduct problems using the Strengths and Difficulties Questionnaire and difference in conduct problems between the lower and higher birth weight twins in birth weight-discordant twin pairs. Results: In birth weight-discordant twin pairs, the twin with the higher birth weight was reported to have more conduct problems compared with the twin with the lower birth weight in 41.1% of twin pairs, whereas the twin with the lower birth weight was reported to have more conduct problems only in 20.5% of twin pairs. This effect tended to be stronger in dizygotic compared with monozygotic twins. Conclusions: The findings suggest an effect of birth weight differences on development of subsequent conduct problems. Further studies are needed to clarify the mediating factors of this effect. The results point to the importance of birth history for subsequent development.