Purpose: The purpose of this paper is to test the effectiveness of a computer-tailored smoking cessation intervention, distributed through 75 Dutch general practices (GP) and 65 pharmacies (PH) in a randomised control trial. Design/methodology/approach: Respondents receive a tailored letter or a thank you letter (control condition). Main outcome measures are a quit attempt, seven-day abstinence and continued abstinence at follow-up (PH: three and 12 months, GP: six months). Effect of use and appreciation of the tailored letter are also tested. Findings: In total, 1,019 smokers are recruited (545 in PH study, 474 in GP study). At 12-month follow-up (PH), respondents receiving the tailored letter report quit attempts significantly more often. No main effects are found in the GP sample. However, reading the tailored letter more than once (GP) and talking with others about the tailored letter (PH) are borderline significant predictors of quitting and point prevalence abstinence. Research limitations/implications: Positive effects of the intervention in a previous efficacy study are not repeated. The possible influence of power, social desirability and testing effect, extraneous events, the availability of the intervention outside the study and data entry issues are presented. Practical implications: Possible explanations for the results found in the study and improvements of the intervention are discussed. This can guide researchers in future smoking cessation intervention development and testing. Originality/value: The paper aims to replicate findings from a previously conducted efficacy study under real-life circumstances. (Contains 2 tables.)