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There is a need for effective smoking cessation interventions that can be disseminated easily in health care and other settings. We previously reported that an extended self-help intervention comprising intensive repeated mailings over 18 months improved abstinence rates through 6 months beyond the end of the intervention when compared to both a reduced version of the self-help materials and a traditional self-help booklet. This report extends the follow-up for an additional 6 months (30-months postbaseline) to examine long-term maintenance of the intervention effect. We hypothesized that the previously observed “dose–response” effect of treatment intensity would be maintained. Participants were randomized to Traditional Self-Help (TSH, n = 638), Standard Repeated Mailings (SRM, n = 614), or Intensive Repeated Mailings (IRM, n = 622). TSH received an existing self-help smoking cessation booklet. SRM received 8 cessation booklets mailed over 12 months. IRM received monthly mailings of 10 booklets and additional material to enhance social support over 18 months. Follow-up assessments occurred every 6 months through 30 months. Data were collected 2010–2013 and analyzed 2014–2017. At 1 year posttreatment, there was a linear dose effect with the highest abstinence rates observed in IRM (33%), followed by SRM (29%), and then TSH (23%; p = .002). Paired comparisons indicated that IRM was superior to TSH (p = .002). Results revealed a robust intervention effect for the intensive self-help intervention that was maintained 12 months after treatment completion. This further supports extended self-help as a low-cost intervention for smoking cessation.