Hartford Dispensary, a private, not-for-profit behavioral health care organization specializing in opioid treatment program services, serves more than 4,200 clients per day in seven licensed facilities. In May 2002, the Dispensary began offering free combined hepatitis A and B vaccine for all eligible clients in two Hartford clinics. Initial eligibility criteria required a client to be hepatitis C antibody positive, hepatitis B virus (HBV) surface antigen (HBsAg) negative, not pregnant, and not have a contraindication due to a medical condition. From May 2002 through April 2005, 5,419 doses of combined vaccine were given. Of the 2,072 clients who received a first dose, 92% (1,914) received the second dose and 69% (1,433) completed the series. Long treatment duration, frequent client visits, availability of medical staff, and counseling support contributed to high rates of combined vaccine series completion in the two programs. Opioid treatment programs appear to be good settings in which to provide hepatitis vaccination for high-risk adults.