Cigarette smoking is the most important preventable cause of death in the United States. Surveys of patients, however, suggest that many physicians do not routinely counsel smokers to quit. Because physicians may not consider counseling against smoking to be as worthwhile as other medical practices, we examined its cost-effectiveness. We based our estimates of the effectiveness of physician counseling on published reports of randomized trials and our estimates of its cost on average charges for physician office visits. Our results indicate that the cost-effectiveness of brief advice during routine office visits ranges from $705 to $988 per year of life saved for men and from $1204 to $2058 for women. Follow-up visits about smoking appear to be similarly cost-effective. Physician counseling against smoking, therefore, is at least as cost-effective as several other preventive medical practices and should be a routine part of health care for patients who smoke.