Objective: To determine tamoxifen (TAM) and its metabolites, endoxifen (END) and 4-hydroxytamoxifen (4-HT) in patients’ DBS simultaneously for monitoring studies purposes. Methods: The UPLC-MS/MS method was developed and validated with clomiphene as the internal standard. Optimization was done by evaluating several parameters that affect the efficiency of DBS preparation and analysis of TAM and its metabolites. Results: Sample preparation was performed by protein precipitation using methanol. The separation was performed on UPLC Class BEH C18 using formic acid 0.1%-formic acid 0.1% in acetonitrile (35:65) as the mobile phase in isocratic mode at 0.25 ml/minute. The mass detection was performed on Waters Xevo TQD using ESI+for TAM, END, 4-HT, and clomiphene as internal standard with m/z value: 372.2>72.27; 374.29>58.2; 388.29>72.19; dan 406.28>100.17 respectively. This method was linear within the range of 5-200 ng/ml for TAM; 1-40 ng/ml for END; and 0.5-20 ng/ml for 4-HT with r value of ≥0.9983; ≥0.9964; and ≥0.9981.%diff and%CV of the assay were within 15% and within 20% for LLOQ. The method was applied to 29 breast cancer patients, where the results lied between 30.29 and 188.63 ng/ml for tamoxifen, 1.45 and 28.77 ng/ml for endoxifen, 0.21 and 11.28 ng/ml for 4-hydroxytamoxifen. Conclusion: This method has successfully fulfilled validation requirement referring to 2011 EMA and 2013 FDA guidelines. The method was successfully applied to determine TAM, END, and 4-HT in DBS of breast cancer ER+patients. TAM and its metabolites level in patients were showed high variability with END concentration of 2 patients below the clinical threshold. © 2019 The Authors.