Background: It has been postulated that endoxifen levels are better predictors of tamoxifen efficacy than CYP2D6 phenotype. Although in a retrospective study an endoxifen threshold of 5.9 ng/ml for efficacy was described, confirmation based on prospective studies is lacking. The objective of the prospective CYPTAM study (NTR1509) is to associate endoxifen levels and, CYP2D6 phenotypes with clinical outcome in early stage breast cancer patients receiving tamoxifen. Methods: Breast cancer patients who were receiving adjuvant tamoxifen were included. Blood samples were used for CYP2D6 genotyping and endoxifen levels by Amplichip and high-performance liquid chromatography-tandem mass spectrometry assay, respectively. Endoxifen levels and CYP2D6 phenotypes were associated with relapse-free survival (RFS) by using Cox-regression analysis. Patients who changed to an aromatase inhibitor, were censored at the time of switch. Results: A total of 667 pre and post-menopausal patients were enrolled. No association was found between endoxifen serum levels used as a continuous variable and RFS (Adjusted Harzard Ratio (HR): 0.991, 95 % CI: 0.946-1.038, p-value: 0.691). Categorizing endoxifen levels in quartiles, or using 5.9 ng/ml as threshold did not alter these results. In addition, no association was observed between CYP2D6 phenotypes and RFS(Adjusted HR: 0.929, 95 % CI 0.525-1.642, p-value 0.799). Conclusions: This first prospective clinical study shows no association between endoxifen levels and CYP2D6 phenotypes with RFS in early breast cancer patients using adjuvant tamoxifen. These results do not support the use of therapeutic drug monitoring based on endoxifen levels or CYP2D6 genotyping. Associations between Endoxifen levels and CYP2D6 phenotypes with RFS. Clinical trial information: NTR1509.