Few family studies of delinquency have focused on siblings. We use a sibling research design to evaluate shared (i.e., family) and unshared environmental influences on delinquency. The 15–22-year-old adolescent siblings were nationally representative, and uniquely, in families of 2 to 4 siblings. No unshared family environmental influences were found for sisters and for mixed-sex siblings, but they may exist for brothers. The data suggested substantial shared environmental and/or shared genetic influences for siblings: the median sibling correlations, averaged over family sizes, were: brothers, r= .30; sisters, r= .28; and mixed sex, r= .21.