Background: Previous studies with adult smokers have shown an association between number of cigarettes smoked per day (CPD) and levels of biomarkers of exposure to the tobacco-specific lung carcinogen 4-(methylnitrosamino)-1-(3- pyridyl)-1-butanone (NNK). This study compared carcinogen and nicotine exposure in adolescent and adult smokers across categories of CPD. Method: Baseline smoking history and biomarker data were merged from six studies to make two samples: one of adolescent smokers and one of adult smokers. Metabolites of NNK, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol (NNAL), and its glucuronides (NNAL-Gluc) and total cotinine were quantified in urine. Results: CPD was stratified into categories of 5 to 10, 11 to 15, and 16 to 20 CPD. Adolescents tended to have lower mean levels of NNAL plus NNAL-Glucs (total NNAL) compared with adults, although differences were not significant overall. Adolescent mean levels of NNAL/CPD were significantly lower than adult levels only in the 11 to 15 CPD category (P = 0.045). However, a significant positive relationship was observed for total NNAL/CPD by age. No significant differences between adolescents and adults were found in mean levels of total cotinine or cotinine/CPD. A subsample of urines from adolescents and adults were analyzed for NNAL-Glucs and NNAL. Adolescents and adults did not significantly differ in the ratio of NNAL-Glucs to NNAL. Conclusions: Adolescent uptake of NNK and nicotine tends to be lower although not statistically different from adults. The lack of significant differences may be due to the wide variation in exposure in adolescents. Some adolescent smokers are exposed to lung carcinogens at levels similar to those of adults.