To minimize impact of injection drug use on HIV/AIDS transmission, known as Harm Reduction, a longitudinal study on methadone maintenance substitution (MMS) trial was conducted in Drug Dependence Hospital, Jakarta in November 2003 and Sanglah Hospital, Denpasar in Januury 2004. Respondents of the study were 58 methadone clients From Jakarta and 22 from Denpasar, as well as 11 RSKO's staffs and seven staffs from Sanglah Hospital. Data were collected by questionnaires, interviews and focus group discussion in the third and sixth month from the client admission. In addition to minimize HIV/AIDS transmission, other benefit of the program is to increase productivity and social participation of the MMS participants. Applying the WHO responsiveness concept for health service performance, we measured the performance of MMS. Both In the first three and six month, patients had positif responsiveness with total and component average score not less than 30. But there was no significant change comparing responsiveness in the third and sixth month. Among the components of responsiveness, information and communication received most positif evaluation from patients, while regulation received the lowest evaluation. Generally, all patients support the MMS program. Staffs of MMS had neutral attitude toward the program, try their best to perform their job and learn to understand more on their clients world. The conclusionis that MMS indeed provide positive impact to the clients, and further understanding from staffs toward their clients is necessary for better communication with the clients.