Cigarette smoking is the most common cause of lung cancer. The greatest risk of lung cancer is among those who started smoking early in life and continued throughout their lives. However, the molecular mechanisms responsible for the susceptibility to lung cancer in the young smoker are not clear. Recently, several groups have reported that the hypermethylation of CpG islands is associated with exposure to tobacco smoke. We studied the association between the age at starting smoking and hypermethylation of p14, p16, and RASSF1A promoters in 204 primary non-small cell lung cancer patients. We also examined whether hypermethylation of the RASSF1A promoter is an independent prognostic factor. Methylation rates in the 204 samples were detected in 9% for p14, 27% for p16, and 32% for RASSF1A. There was no relationship between the hypermethylation of p14 and p16 and the age at starting smoking. However, hypermethylation of the RASSF1A promoter was found to be significantly associated with the age at starting smoking (P = 0.001). No relationship was found between the methylation status of the RASSF1A promoter and other smoking variables, such as pack-years, smoking status, and the duration of smoking. The age at starting smoking in patients with hypermethylation of RASSF1A was earlier than that of patients without hypermethylation of the RASSF1A promoter (19 +/- 8 versus 25 +/- 7; P = 0.001). Young smokers who started smoking before age 19 were 4.23 times [95% confidence interval (CI) = 1.03-9.67; P = 0.001] more likely to have hypermethylation of the RASSF1A promoter than smokers who started smoking after the age of 19. Furthermore, hypermethylation of the RASSF1A promoter was found to be associated with a poor prognosis in non-small cell lung cancer patients at stages 1 and 2 (P = 0.02 and 0.01, respectively; Log-rank test). The hazard of failure was approximately 3.27 times higher for patients with hypermethylation of the RASSF1A promoter than for those without hypermethylation of the RASSF1A promoter (95% CI = 1.42-8.71; P = 0.01). Young smokers who started the habit before the age of 19 also had a poorer prognosis than those who started after the age of 19 (hazard ratio = 2.14, 95% CI = 1.22-9.11; P = 0.02). Our results suggest that starting cigarette smoking at an early age is associated with hypermethylation of the RASSF1A promoter and that hypermethylation of the RASSF1A promoter may be an independent prognostic factor in primary non-small cell lung cancer.