The mutant of CYP2D6*3 allele with A 2637 deletion in exon 5 and the mutant of CYP2D6*4 allele G 1934 →A, splice site defect are among the most common polymorphic alleles of CYP2D6 gene, resulting in a decreased or no activity of CYP isoenzyme. In this study, a reliable polymerase chain reaction-restriction fragment length polymorphism method for identification of CYP2D6*3 and CYP2D6*4 alleles was used to investigate the genotype and phenotype prevalence in the groups of normal controls, and of cirrhosis and cancer patients. The results showed none of 36 controls genotyped for 2D6*3 and 2D6*4 allele to have the 2D6*3 allele with frameshift mutation in exon 5, while 33 % (n=12) were found to bear the 2D6*4 allele with G to A mutation at the intron 3—exon 4 junction. In breast cancer patients (n=35) genotyped for 2D6*3 and 2D6*4 alleles, none with 2D6*3 allele was found either, but 60 % (n=18) were found to bear the 2D6*4 allele. In patients with head and neck squamous cell cancer, there was only one subject with 2D6*3 allele and he was heterozygous. Among them, as many as ten (40 %) patients were found to bear 2D6*4 allele. In the cirrhosis group, none of the patients was found to have the 2D6*3 allele, while the CYP2D6*4 allele was found in 23 % (n=6) patients. The phenotype predicted according to the genotype was as follows: in the control group, 3% of individuals were identified as poor metabolizers, 70 % as extensive metabolizers, and 27 % as heterozygote extensive metabolizers. In the group of breast cancer, 7% of the patients were identified as poor metabolizer, 57 % as extensive metabolizer and 36% as phenotype. In squamous cell cancer and cirrhosis patients, the incidence of poor metabolizer was zero, and of heterozygotes extensive metabolizer 42 % and 31 %, respectively.