The prevalence of CYP2D6*3 and CYP2D6*4 alleles in normal controls and cancer patients was studied using the reliable PCR-SSCP method. In the control group (n=144), four subjects (2.8%) were found to carry CYP2D6*3 allele (heterozygote), while 30 (20.8%) subjects carried CYP2D6*4 allele (18.8% heterozygotes, 2.1% homozygotes). One (1.3%) of the breast cancer (BC) patients (n=76) carried CYP2D6*3 allele, but 24 (31.6%) carried CYP2D6*4 allele (26.3% heterozygotes, 5.3% homozygotes). In the head and neck cancer (HNC) group (n=56), two (3.6%) patients were heterozygous for CYP2D6*3 mutation and 15 (26.8%) for CYP2D6*4 mutation. Fourteen of 56 (25%) and one of 56 (1. 8%) of these patients carried heterozygous and homozygous mutations, respectively. In controls, 2.1% were identified as poor metabolizers (PM), 76.4% as extensive metabolizers (EM), and 21.5% as intermediate heterozygotes (IEM). In BC group, 5.3, 27.6 and 67.1% were classified as PM, IEM and EM, respectively. In HNC group, the incidence of PM was 1.8, but as many as 28.6% were identified as IEM phenotypes.