Nicotine replacement by transdermal patches is more effective than placebo in smoking cessation, but has a low success rate after one year (9-18%). We tested whether this was attributed to insufficient nicotine replacement. We conducted a randomized trial to investigate the effect on outcome of different doses of transdermal nicotine replacement after stratification according to baseline plasma cotinine values. Two hundred and ninety seven adult smokers were enrolled. Those with baseline cotinine < or = 250 ng.ml-1 (low cotinine) were randomly assigned to placebo (LC-P) or to 15 mg 16 h nicotine patches (LC-15), and those with baseline cotinine > 250 ng.mL-1 (high cotinine) were randomly assigned to 15 mg (HC-15) or 25 mg (HC-25) 16 h nicotine patches. Plasma nicotine and cotinine values, expired carbon monoxide and withdrawal symptoms were measured at scheduled intervals during treatment. Smokers in the LC-15 group had a significantly higher success rate than placebo (28 vs 9%). Smokers with high baseline cotinine had lower success rates, and a high dose of nicotine did not increase success rate (HC-25 9% vs HC-15 11%). Subjects in the HC-15 group had the lowest percentage of nicotine replacement and a higher prevalence of withdrawal symptoms than the HC-25 group. Replacement was similar in groups LC-15 and HC-25, but the success rate was significantly lower in HC-25 group, despite similar levels of withdrawal symptoms. We conclude that a higher success rate was obtained after one year in smokers with low baseline plasma cotinine values. Determination of plasma cotinine values may be, thus, helpful in identifying smokers who could benefit from transdermal nicotine replacement.