In a systematic review of the efficacy of interventions intended to help people stop smoking, data have been analyzed from 188 randomized controlled trials. Following personal advice and encouragement to stop smoking given by physicians during a single routine consultation, an estimated 2% (95% confidence limits, 1%, 3%; P < .001) of all smokers stopped smoking and did not relapse up to 1 year as a direct consequence of the advice. The effect is modest but cost-effective: the cost of saving a life is about $1500. Supplementary interventions (follow-up letters or visits, demonstration of spirometry, etc) have an additional effect--variable in extent. Advice and encouragement are particularly effective for smokers at special risk--pregnant women (efficacy; 8%) and patients with ischemic heart disease. Behavior modification techniques (relaxation, rewards and punishment, avoiding "trigger" situations, etc), in group or individual sessions led by a psychologist, have an effect that is statistically significant (P = .05) but no greater than simple advice by a physician (2%); yet, these techniques are several times more expensive. The effect of hypnosis is unproved (no trials have used biochemical markers). Nicotine replacement therapy is effective in an estimated 13% of smokers who seek help in cessation; the effect is greater in those who are nicotine-dependent. Other pharmacological treatments are not of proven efficacy, and acupuncture is ineffective. Sudden cessation or gradual reduction in smoking are similar in their efficacy on average. Physicians should take time to advise all their patients who smoke to quit. Smokers who are intent on stopping should be given additional support and encouraged to use nicotine replacement therapy.