The transdermal nicotine patch has proved an effective aid to smoking cessation. The ease of securing good compliance gives it a potential advantage over nicotine gum as an adjunct to brief advice and support in primary care settings where the major public health impact is obtained. In a preliminary report of half the sample of a randomized placebo controlled trial, we showed the patch to be effective in a general practice setting. We report here the definitive results of the full sample, including dose effects, predictors of outcome and other issues of theoretical and practical interest. A total of 1200 heavy smokers (> or = 15 per day), attending 30 general practices in 15 English counties received brief GP advice, a booklet and 16 hours per day patch treatment for 18 weeks. Dose increase and abrupt vs. gradual reduction of patch dosage were also randomized and follow-ups conducted at 1, 3, 6, 12, 26 and 52 weeks. Outcome was measured by self-reported complete abstinence from week 3 to 52 with biochemical validation at all follow-up points. Nicotine patch treatment doubled the rate of continuous abstinence up to 1 year (nicotine 9.6%, placebo 4.8%, p < 0.01); it most likely worked by reducing withdrawal symptoms. It enhanced cessation during the first week and reduced relapse during the second week. The dose increase after week 1 produced no sustained increase in cessation. Gradual reduction was no better at preventing relapse than abrupt withdrawal of patches after week 12. Whether relapse would have increased by ending treatment at some point between weeks 3 and 12 was not tested. Although pre-treatment dependence on cigarettes was prognostic of failure, the patches were equally helpful to both highly and less dependent smokers. Patches were particularly helpful to smokers with pre-treatment subclinical dysthymic symptoms. All but one of the 96 subjects eventually achieving long-term abstinence in the study quit during the first week of cessation.