To verify the usefulness of CYP2D6 genotyping for the prediction of tamoxifen metabolism impairment, i.e., not achieving therapeutically beneficial (Z)-endoxifen exposure, we measured the steady-state concentration of tamoxifen and its metabolites in the plasma of nearly 280 Polish patients with breast cancer that were routinely treated with the standard 20 mg daily dose of the drug. This evidence concerns the gene CYP2D6 and breast cancer.