BACKGROUND: Having a family member with substance use problems affects family functioning, which may lead to increased medical problems and increased health care utilization and costs in the other family members. AIM: We sought to estimate the excess medical costs and prevalence of diagnosed health conditions of family members of persons with an alcohol or drug diagnosis (AOD) compared with the family members of similar persons without an AOD. METHODS: Using a large health plan’s administrative databases, we identified persons who received an AOD between 2001 and 2004 and a similar group of persons with no AOD during that time. Using a hierarchical linear mixed model, we compared the cost and utilization of the family members of the AOD and non-AOD patients in the 2 years prior to the AOD patient’s first AOD. Using logistic regression, we determined whether the family members of patients with AODs were more likely than comparison family members to be diagnosed with medical conditions. RESULTS: Family members of patients with AODs had greater health care costs than comparison family members in the second year before the index date (490 dollars) and in the year before the index date (433 dollars). This was the case for both adult and child family members. They also were more likely to be diagnosed with many medical conditions, especially substance abuse and depression. CONCLUSIONS: Family members of patients with AODs have greater health care costs and are more likely to be diagnosed with a number of medical conditions than family members of similar persons without an AOD.