To assess and compare the impact on quit attempts of online computer-tailored smoking cessation counseling reports and untailored reports, we performed a randomized controlled trial on a smoking cessation website in 2007-2008. After answering a questionnaire, current and former smokers were randomly assigned to immediately receiving either an online, individually tailored counseling report or a personalized but untailored generic report. Participants were invited by e-mail to report any smoking in the previous 24 hours, 48 hours after baseline. We used an intention-to-treat analysis, where nonrespondents at follow-up were counted as smokers. There were 2,872 participants at baseline and 2,226 at follow-up (78%). At baseline, there were 76% of current smokers (mean = 18 cigarettes/day) and 24% of recent quitters (median = 7 days of abstinence). The same proportion of smokers in both study groups had made a 24-hour quit attempt at follow-up (12.1%, P = 1.0). In baseline recent quitters, lapse/relapse rates at follow-up were similar in both groups (tailored: 25.1%, untailored: 23.5%, P = 0.64). We conclude that untailored reports were as effective as tailored reports in the short term. Even though these particular computer-tailored reports were not more effective than untailored reports, meta-analyses show that computer-tailored documents are in general more effective than untailored ones.