Classification tree analysis of environmental and genetic risk factors for lung cancer showing cut-off values for cigarette packs a year, education years, body mass index, chronic obstructive disease history, and CYP1B1 C/G or G/G genotype. The target variable is the prevalence and the analysis produces nine terminals. The prevalence in the entire population was 65.7% (339/516), and the first split is performed on cigarette packs a year with a split point of 18.5 packs/year. This produces two subgroups with respective prevalence of 24% (29/119) and 78.1% (310/397).