Based on pharmacological and genetic studies suggesting a role of the serotonergic system for nicotine dependence, two genetic variants, one on the tryptophan hydroxylase 1 (TPH1) and one on the TPH2 gene, were investigated. The TPH2 -703G/T promoter polymorphism was associated with smoking status and age of smoking onset in two independent Caucasian samples of different age cohorts. Sample 1 consisted of 3,602 subjects of a population-based cohort study of whom 95% were 40-65 years old, and sample 2 consisted of 723 subjects in the twenties. The TPH1 779A/C was only investigated in sample 2 where additional data with respect to the degree of nicotine dependence were assessed. Results yield support for previous findings of an association between the AA genotype of TPH1 779A/C and nicotine status and a tendency for a heterosis effect with respect to the degree of nicotine dependence. The TPH2 -703G/T was significantly associated with age of smoking onset in both samples. The interaction between the T allele and gender was significant in both samples. Carriers of the GG genotype started significantly earlier to smoke than carriers with a T allele. In the older cohort, age of onset was earlier in carriers of the GG genotype only in men, and in the younger cohort the GG genotype predisposes only women to start smoking earlier in life. This interaction could constitute a gene-by-environment interaction that is discussed on the background of previous studies relating the -703G/T polymorphism to anxiety, a personality dimension that has been shown to be a risk factor for nicotine dependence.