Self-efficacy ratings coincided with illicit opioid use across the 3 phases of a 180-day methadone detoxification treatment. Efficacy ratings increased after patients received their first dose of methadone, did not change while they were maintained on a stable dose of methadone, and declined during the taper as they attempted to face high-risk situations without the full benefit of methadone. Efficacy ratings measured at a point before a phase of treatment predicted illicit opioid use across that phase. For clarification of the relation between self-efficacy and illicit opioid use, 3 conceptual models proposed by J.S. Baer, C.S. Holt, and E. Lichtenstein (1986) were tested. Self-efficacy influenced subsequent drug use in parallel with previous behavior, but this influence was found only at the start of the stabilization phase and immediately before the start of the taper phase. These findings highlight the usefulness of the self-efficacy concept for the treatment of opioid addiction.