Interim medication-only treatment has been suggested for the initiation of opioid maintenance treatment (OMT) in opioid-dependent subjects, but this rarely has been studied using buprenorphine instead of methadone. Following a pilot trial assessing interim buprenorphine-naloxone treatment in order to facilitate transfer into OMT, we here aimed to study retention, and potential correlates of retention, in full-scale treatment. Thirty-six patients successfully referred from a waiting list through an interim treatment phase were followed for nine months in OMT. Baseline characteristics, as well as urine analyses during the interim phase and during full-scale OMT, were studied as potential correlates of retention. The nine-month retention in OMT was 83 percent (n = 30). While interim-phase urine samples positive for benzodiazepines did not significantly predict dropout from full-scale OMT (p = 0.09), urine samples positive for benzodiazepines within full-scale OMT were significantly associated with dropout (p < 0.01), in contrast to other substances and baseline characteristics. Retention remained high through nine months in this pilot study sample of patients referred through buprenorphine-naloxone interim treatment, but use of benzodiazepines is problematic, and the present data suggest that it may be associated with treatment dropout.