The aim of this study was to investigate the association the Val158Met polymorphism of the catechol-O-methyltransferase (COMT) gene and predisposition to alcoholism and heroin dependence. The authors genotyped DNA samples from 964 Russian males (395 alcoholics, 243 heroin addicts and 326 healthy controls). The association between the Val158Met COMT polymorphism and alcoholism was found in males with high density of family history (two or more blood relatives with alcoholism within the family). In this group, the frequency of a L (Met) allele was significantly higher in comparison with controls (p=0,001), patients without family history (p=0,034) and patients with the mild density of family history (p=0,0005). The frequency of the HH (ValVal) genotype was reduced as well compared to the controls (p=0,003). In the group of heroin addicts with the mild density of family history, there was a trend to lower frequency of the HH genotype (p=0,058) compared to patients without family history. The results suggest that the functional Val158Met COMT polymorphism is one of the significant markers of genetic predisposition to addiction diseases.