The Opiate Treatment Index (OTI), an instrument designed to monitor treatment outcome of opioid users, is becoming increasingly popular among clinicians and researchers in the United Kingdom. This study was designed to examine how the OTI would perform when administered by clinic staff compared to externally contracted researchers in clinical settings. In a confidential setting, the OTI was administered twice to 55 opioid users from two London clinics, in a random fashion, once by trained clinic staff and once by researchers within a 2-week period. The data generated by both groups were similar with respect to social functioning, physical health and psychological adjustment. Where differences occurred, in almost all the cases they were not statistically significant. Clients reported slightly higher levels of drug use episodes, injecting behaviour and criminal activity to researchers. In both groups, none of the clients admitted to paid sex, and low levels of criminal activity and illicit drug use are reported-findings which are most probably related to the stability of these patients rather than systematic under-reporting. Although this cross-sectional study showed that the OTI could be applied equally effectively by clinic staff and researchers in clinical settings, further research is needed to examine whether the situation would hold true in routine outcome monitoring. To ensure that reliable and valid data are generated in routine monitoring of treatment programmes, several issues relating to clinic staff (e.g. motivation, time); clients (e.g. co-operation, confidentiality) and researchers (e.g. cost) need to be addressed.