This study evaluated the effect of telephone counseling as an adjunct to a self-help program for smoking cessation conducted through the mail. We obtained demographic and consumption information on those smokers who requested participation in the study. These participants (N = 200) were randomized into two study groups: (1) the standard self-help group (n = 100) (median age: 35.1 years; pretreatment consumption of 28.0 cigarettes/day); (2) the self-help group receiving additional multiple-contact telephone counseling (n = 100) (median age: 36.7 years; pretreatment consumption of 27.3 cigarettes/day). At the 12-month follow-up, the carbon monoxide in expired air was used to distinguish nonsmokers from smokers. Significant differences were found in the rates of continuous abstinence in both groups for each period evaluated. In the standard self-help group, the continuous abstinence rate at the 3-month follow-up was 21%, 18% at the 6-month follow-up, and 14% at the 12-month follow-up. The telephone counseling group yielded a 48% continuous abstinence rate at the 3-month follow-up, 40% at the 6-month follow-up, and 27% at the 12-month follow-up. The results of this randomized controlled trial show that telephone counseling was an effective aid for the smoking cessation program.