This study had a limitation. As all patients were recruited through specialty treatment services, the sample could not be considered a reflection of patients with alcohol or opiate dependence in routine medical practice. If the study revealed that the measure could produce reliable and valid assessments of individuals, certain properties, such as responsiveness, were not determined and might be evalu- ated in a future study. It is relevant to note that the Q-LES- Q-SF appeared to discriminate QoL more precisely according the type of substance in this study than did the SF-12 although the latter instrument has been used in numerous studies of patients with alcohol or opiate dependence. Similarly, although the SF-12 and the original Q-LES-Q form have been used for longitudinal repeated measures in QoL assessments among substance abusers, future research might be based on the Q-LES-Q-SF, in particular using IRT, allowing an exploration of whether the answer to a questionnaire item could be influenced by patient characteristics.