This study examined aspects of the sibling relationship that predicted helping profession choice and volunteerism in siblings of individuals with mild intellectual deficits at 3 points in adulthood: their mid-30s, early 50s, and mid-60s. The 393 respondents were from the Wisconsin Longitudinal Study, a prospective, longitudinal study following participants from ages 18 to 64 years. Being an only sibling predicted greater helping profession choice for female but not male siblings. Being older than the brother or sister with mild intellectual deficits as well as having more contact with and feeling closer to that brother or sister predicted more volunteerism for female but not for male siblings. Earlier measures of contact and closeness were better predictors of volunteerism than concurrent measures.