The study investigates patient preferences and beliefs and treatment program factors related to the decision to prescribe either buprenorphine or methadone to opiate-dependent patients. The sample (N = 192) was recruited from 10 addiction treatment services in London. Data were collected by means of a single structured interview conducted with patients commencing a treatment episode at the participating agencies. Data on patient demographics, beliefs, attitudes, and preferences were collected using a structured interview. Data regarding treatment goals and prescribed medication were collected from interviews with clinical staff. Oral methadone had a higher preference rating than buprenorphine. Clinical prescribing practices were influenced by patient preferences (both positive and negative), by prior treatment experiences, and by current treatment goals. Patient preferences and beliefs about opioid agonist medications served as an important influence upon clinical prescribing practices. The odds of being prescribed buprenorphine were three times greater among those patients who reported a preference for buprenorphine. The odds of receiving a prescription for methadone were about twice as great among those for whom methadone was the more preferred medication. Preferences were related to previous treatment experiences with these opioid agonists, and for patients in both groups, personal experience was the most important source of information about the treatment options. Buprenorphine was more likely to be prescribed for short-term detoxification and methadone for maintenance treatment.