Aim To assess the effectiveness of a new computer-generated tailored advice 1 programme designed to be used by smokers and recent quitters having problems staying stopped. Design Randomized trial comparing a series of questionnaire assessments leading to tailored computer-generated advice letters mailed at strategically relevant times, to a no extra treatment control sent standardized printed self-help materials. Setting Victoria, Australia. Participants A total of 1058 smokers or recent quitters recruited from callers to the Quitline. Measurements Smoking status and sustained abstinence Elf 12-month follow-up, plus extent of participation in the intervention. Findings Using a conservative analysis (missing data coded as a treatment failure), 6-month sustained abstinence was reported by significantly more participants in the computer-generated tailored advice (20%) than the standard printed materials condition (12%) at 12-month follow-up OR 1.82 (1.31-2.55)). Group differences in point prevalence abstinence (28% intervention, 25%, control) were not significant. Among participants in the tailored advice condition, 6-month sustained abstinence was associated with the number of advice letters received. Conclusion The provision of a series of tailored, computer-generated advice letters resulted in greater rates of sustained cessation than for controls. A dose-response relationship was found, with increased compliance with the intervention associated with improved cessation outcomes. The programme appears to have much of its effect by preventing relapse.