590 Background: Polymorphisms in cytochrome P450 2D6 gene affect the plasma concentration of tamoxifen active metabolites (endoxifen). Some drugs are known to be CYP2D6 inhibitors. We aim to determine the relationship between CYP2D6*4 polymorphisms, concomitant CYP2D6 inhibitors use and clinical outcomes of breast cancer patients receiving adjuvant tamoxifen (TAM). Patients and Methods: CYP2D6*4 (1934 G>A+1) genotype was determinated from DNA of blood samples using PCR-RFLP technique and Taqman Allelic Discrimination Assay in a series of 84 breast cancer patients receiving adjuvant TAM. CYP2D6 inhibitors were recorded. Chi-square test and logistic regression models were used to determinate association between genotype, use of concomitant CYP2D6 inhibitors and disease relapse rate. Results: In our 84 patients series mean age was 51.5y. (33–71). 14.8% were stage I, 58.0% stage II and 27.2% stage III. 61.4% were nodes positive and 98.7% tumors had positive hormonal receptors. We observed disease recurrence i...