To characterize prescription opioid dependent patients in a methadone maintenance treatment (MMT) program, a detailed retrospective chart review of new admissions (1997-1999, n=178, mean age=34.5+/-0.7 years, 65% male) was conducted. At admission most patients (83%) had been using prescription opioids (+/-heroin). Four groups were identified: 24% had used prescription opioids only; 24% used prescription opioids initially and heroin later; 35% used heroin first and prescription opioids subsequently; and 17% had used heroin only (this group was significantly younger: mean age 26+/-1 years, P=0.0001). Subjects reported regular use of prescription opioids at higher than therapeutic dosages. For example, in the 'prescription opioid only' group the reported mean (+/-S.E.) number of codeine or oxycodone-containing tablets consumed daily was 23 (+/-6) tablets and 21 (+/-3) tablets, respectively. There were no significant differences found amongst the groups in measures of social stability. Those dependent on prescription opioids alone were less likely to use illicit non-opioid drugs or to be associated with injection drug use. Those that used prescription opioids only or initially were more likely to have ongoing pain problems and to be involved in psychiatric treatment. Further research is required to better elucidate the complex relationships between pain, mental health and addiction in order to develop optimal prevention and treatment strategies for prescription opioid dependence.