The estrogen receptor α (ERα) status of breast tumors is used to identify patients who may respond to endocrine agents such as tamoxifen. However, ERα status alone is not perfectly predictive, and there is a pressing need for more reliable markers of endocrine responsiveness. In this aim, we used a two-step strategy. We first screened genes of interest by a pangenomic 44 K oligonucleotide microarray in a series of ten ERα-positive tumors from five tamoxifen-treated postmenopausal patients who relapsed (distant metastasis) and five tamoxifen-treated postmenopausal patients who did not relapse, matched with respect to age, Scarff–Bloom–Richardson grade, lymph node status, and macroscopic tumor size. Genes of interest (n=24) were then investigated in an independent well-characterized series of ERα-positive unilateral invasive primary breast tumors from postmenopausal women who received tamoxifen alone as adjuvant hormone therapy after primary surgery. We identified four genes (HRPAP20, TIMELESS, PTPLB, and MGC29814) for which high mRNA levels were significantly associated with shorter relapse-free survival (log-rank test). We also showed that hormone-regulated proliferation-associated 20 kDa protein (HRPAP20) and TIMELESS are 17β-estradiol-regulated in vitro and are ectopically expressed in OH-Tam-resistant cell lines. In conclusion, these findings point to HRPAP20 and TIMELESS as promising markers of tamoxifen resistance in women with ERα-positive breast tumors.