A transdermal nicotine patch, which delivers 0.7 mg/cm2 per 24 h and is available in sizes of 10, 20, and 30 cm2 was tested in subjects from 21 general medical practices in a 3-month, placebo-controlled randomised double-blind study. The nicotine group (n = 100) and the placebo group (n = 99) were similar at entry. Participants who smoked more than 20 cigarettes a day were treated with the 30 cm2 patch and the others with the 20 cm2 patch. When abstinence, defined as smoking 0-3 cigarettes per week and verified by CO measurement, was achieved, the next smallest patch was applied. After 1, 2, and 3 months of treatment 41, 36, and 36%, respectively, in the nicotine group were abstinent. The corresponding figures in the placebo group were 19, 20, and 23%. The differences were significant for all 3 months. Body weight did not increase in the nicotine group, but in the placebo group the mean increase was 4.4 kg. Craving and withdrawal symptoms decreased more with nicotine substitution for cigarettes. The patches were generally well tolerated, although 25% of subjects in the nicotine group and 13% in the placebo group had transient local erythema after application of the patch; 5 members of the nicotine group withdrew because of poor cutaneous tolerance.