Results are presented of a randomized field trial comparing two aftercare regimes, namely individual versus group delivery of a structured relapse prevention approach. Two addictions treatment programs (one a 12-Step 26-day residential program, the other an evening group counselling program) implemented structured relapse prevention in either group or individual format as part of the first three months of aftercare. Process measures (e.g. attendance, client satisfaction) indicated that both group and individual formats were delivered very successfully at both sites. Follow-up rate at 12 months across both programs was 74%, and drinking and drug use at the 12-month follow-up was substantially less than use at entry into treatment. However, there were no significant differences in outcomes between individual and group delivery on any of the alcohol or drug use measures. Only one psychosocial outcome measure (social support from friends at 12-month follow-up) showed a significant difference for format and it favored the group format. These findings suggest some important directions for future research.