Background: The incidence of CYP2D6*4 among Caucasians is estimated up to 27%, while it is present in up to 90% of all poor metabolizers within the Caucasian population. The hypothesis under question is whether the presence of one or two non-functioning (null) alleles predicts an inferior outcome in postmenopausal women with breast cancer receiving adjuvant treatment with tamoxifen. The aim of the present study is to estimate the incidence of CYP2D6*4, in the Greek population and more precisely among females suffering from breast cancer. Materials and Methods: Eighty unrelated mainland Greek female volunteers suffering from hormone-sensitive breast cancer were recruited during their primary handling or follow-up examination in order to provide samples for purification and polymerase chain reaction/ restriction fragment length polymorphism (PCR-RFLP) of genomic DNA derived from buccal swabs. Results: The incidence of individuals with at least one present allele*4 within the Hellenic population was estimated to be as high as 30% (n = 24/80), with a 95% confidence interval of 20% to 40%. From the statistical point of view, it can be securely stated that incidence of *4 among Greek women is over 20%. The incidence of homozygous carriers of *4 in the present sample occurred in 8.75%, while the incidence of allele*4 haplotype occurred in 19.4% (n=160). Conclusion: Although the outcoming results for Greek women are actually in line with existing data for other European nations, it should be noted, that a routine CYP2D6 testing of women suffering from breast cancer is formally not recommended, as the clinical significance of CYP2D6 phenotype in treatment and outcome of breast cancer remains unclear.