AbstractThe goal of this study was to identify factors associated with six- and 12-month retention in methadone maintenance treatment programs (MMTPs) in Massachusetts and Connecticut. Data was obtained from 674 participants, clinic records, and clinic staff. Ethnographic and logistic regression analyses were conducted. Overall, 69% and 48% of the clients remained in treatment at six months and 12 months, respectively. The MMTPs were categorized as either a 12-Step, case management, or primary care model. Factors independently associated with retention in treatment at six months were each one-year increase in age of client (OR 1.05), injecting at three months (OR 0.47), and enrollment in the primary care model (OR 2.10). The same factors were associated with 12-month retention in treatment. To retain clients in MMTPs—which should, in turn, help reduce drug use and prevent HIV transmission among IDUs—younger IDUs and clients still injecting at three months after entering drug treatment may need additional services from the staff, or alternative treatment regimens. MMTP directors should consider differences between these programs and, if appropriate, make changes to increase retention in treatment.