The purpose of this study was to explore the association of a tryptophan hydroxylase gene polymorphism (TPH1 A218C) with the age of alcoholism onset in a Korean population. The genotype and allele frequencies of TPH1 were investigated in 182 male hospitalized patients who met Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR) criteria for alcohol dependence. Alcoholics with the TPH1 AA or AC genotypes had an earlier age of disease onset (median age of onset, 26.5 years) than those with the CC genotype (median age of onset, 30 years; p=.002). Age of onset has been used in classifying alcoholics. The TPH1 polymorphism may explain, in part, the biological basis for these typologies.