We studied the effectiveness of chewing gum containing nicotine, in combination with group counseling, for subjects who were attempting to stop smoking. We used the Horn-Russell scale, based on a smoking questionnaire, to measure dependence on cigarettes; 173 smokers were grouped as highly dependent on nicotine or as having medium to low degrees of dependence. In a randomized double-blind study, the 60 highly dependent smokers were given gum containing 4 mg of nicotine (n = 27) or 2 mg of nicotine (n = 33), and the 113 smokers with medium or low dependence were given gum containing 2 mg of nicotine (n = 60) or a placebo gum (n = 53). All smokers took part in group counseling. In the highly dependent group, abstinence from cigarettes was chemically verified after six weeks, one year, and two years; 81.5 percent, 44.4 percent, and 33.3 percent of the subjects given gum containing 4 mg of nicotine were abstinent after those follow-up periods; the rates of abstinence were 54.5 percent, 12.1 percent, and 6.1 percent, respectively, for the subjects given gum containing 2 mg of nicotine. In the group with medium or low dependence, the rates of abstinence after the same periods were 73.3 percent, 38.3 percent, and 28.3 percent for the subjects given gum containing 2 mg of nicotine and 41.5 percent, 22.6 percent, and 9.4 percent for those given placebo gum. The differences in outcomes were significant at the 5 percent level for all comparisons, with the exception of the 2-mg nicotine gum versus the placebo gum at one year. This study indicates that the effectiveness of nicotine gum is not due to a placebo effect and that it is related to dose. The use of nicotine gum in appropriate doses should be helpful to persons who are attempting to stop smoking.