Substance abusers are at high risk for hepatitis C (HCV) infection and also constitute a group that is medically underserved and hard to reach. We conducted a nationwide survey with 445 randomly selected drug treatment units in the United States to determine unit and patient characteristics associated with the provision of on-site medical services for HCV-infected drug users. Eighty-four percent of the 322 units that estimated having at least one HCV-infected patient reported that they provided patients with HCV-related medical care. Drug treatment units were more likely to provide at least some of this care on site if they were residential, part of a network, or affiliated with a hospital; had medical staff; and required that their patients undergo a medical examination before entering treatment. Some organizational factors appear to influence the provision of on-site medical services to HCV-positive patients in drug treatment units. Further research on the role of such factors could inform the development of effective models of care for patients with hepatitis C in drug treatment organizations.