In the current study, data were taken from a research project that focused on the methodological adequacy of self- reported measures of delinquent behavior. Two measures of family size were used--number of brothers and sisters who lived at home with the subject and total number of brothers and sisters. Findings did not support explanations of the relationship between family size and juvenile delinquency that were based on demographic and background characteristics such as birth order and ethnicity or on social control theory. Exposure to sibling delinquency appeared to satisfactorily explain the effects of family size on self-reported delinquency. Adolescents living in large families were at greater risk of having a delinquent sibling, a finding consistent with propositions derived from social learning theory. 30 references and 3 figures