The authors examined the effect of 24-hour nicotine patches in smoking cessation among over-the-counter customers in Denmark, based on a randomized double-blind placebo-controlled trial. Participants were consecutive customers to whom nicotine patches were offered as the only treatment. Forty-two pharmacies in the areas of Aarhus and Copenhagen in Denmark participated in the trial, and 522 customers who smoked 10 or more cigarettes per day were randomized to either nicotine patches or placebo from January to March 1994. Customers with chronic diseases and pregnant or breastfeeding women were excluded from the trial. Twenty-four-hour patches were offered free of charge during a 3-month period. Those smoking 20 or more cigarettes per day started on a dose of 21-mg/day patches. Customers who smoked less started on patches of 14 mg/day; and for all of the participants, the dose was gradually reduced to 7-mg/day patches during the study period. Smoking behavior and compliance were recorded by means of self-administered questionnaires and telephone interviews. Smoking status was recorded in intervals of 4 weeks, which was fixed to be a treatment period, and 26 weeks after inclusion. There was a significant increase in smoking cessation rates after 8 weeks of follow-up but only among smokers who started on 21-mg/day patches. There was a marked placebo effect at each time of contact during the trial, especially in those smoking fewer than 20 cigarettes per day. Although the noncompliance rate was high overall due to discontinuation in the use of patches by relapsed smokers, noncompliance among successful quitters was low. More side effects were seen in the nicotine group than in the placebo group, but none of the reported side effects were serious. It appears that regular healthy smokers who were customers of nonprescribed nicotine patches and who received 21-mg/day nicotine patches benefited from the active treatment (44.1% stopped smoking after 4 weeks), but almost as many stopped smoking in the placebo group (37.3% after 4 weeks). No significant differences in smoking cessation rates were seen among smokers who started with the low-dose nicotine or placebo patches. Am J Epidemiol 1997; 145: 309–18.