As part of a clinical trial investigating the level of nicotine replacement with different doses of transdermal therapy for smoking cessation, peak and trough serum nicotine and plasma cotinine concentrations were measured in 70 subjects while they were actively smoking (baseline) and daily for 6 consecutive inpatient days while they were receiving transdermal nicotine. Subjects were randomly assigned to a daily 24-hour patch delivering a transdermal nicotine dose of 0, 11, 22, or 44 mg and stratified by self-reported smoking rate as either light (10-15 cigarettes per day), moderate (16-30 cigarettes per day), or heavy (>30 cigarettes per day). Steady-state concentrations of nicotine and cotinine were attained in 1 and 3 days, respectively, at all doses and were independent of baseline smoking rate. Mean percentage replacement of nicotine was calculated by dividing steady-state peak nicotine or cotinine concentrations by their respective baseline concentrations. Significant underreplacement occurred in subjects receiving the 11 mg/day patch regardless of baseline smoking rate. Underreplacement also occurred in moderate and heavy smokers receiving 22 mg/day and in light smokers at this same dose. Complete replacement occurred only in subjects receiving the 44 mg/day patch. These results have several implications for transdermal nicotine therapy. First, with the higher nicotine and cotinine levels observed with heavier smoking, it is inherent that one size does not fit all, and there is a need to consider more individualization of dosage for nicotine patch therapy. Second, there is substantial underreplacement with the 22 mg/day dose in moderate to heavy smokers and in some light smokers. Third, even with twice the usual dose (i.e., 44 mg/day), there was no accumulation of either nicotine or cotinine. Plasma cotinine levels after achievement of steady state (i.e., after 3 days of patch therapy) can be collected at any time and used to calculate percent replacement using baseline levels.