Growth factor signaling and angiogenesis may promote endocrine-resistance in breast cancer and blocking these pathways can overcome resistance in preclinical models. We conducted a phase-II study of adding the VEGFR/Ras/Raf/MAPK inhibitor sorafenib to endocrine therapy in metastatic ER-positive breast cancer, either upon progression or after maximal response with measurable residual disease. Tumor biopsies and serum were collected on days 1 and 28. Primary endpoint was response by RECIST after 3 months and secondary endpoints included safety, time to progression (TTP), and biomarker assessment. Planned sample size was 43 patients but the study closed after 11 patients because of slow accrual. 8 patients had progressive disease (PD) on entry and 3 had stable disease (SD). One patient with SD discontinued sorafenib after 2-weeks because of grade 3 rash. Of the 10 remaining patients after adding sorafenib, 7 had SD (70%), 3 had PD (30%) and median TTP was 6.1-months. Of the 8 patients who entered the study with PD on endocrine therapy, 5 converted to SD (62%) with a median TTP of 6.4-months. Notably, patients on tamoxifen had a median TTP of 8.4-months. The most common adverse events were hypophosphatemia, hypokalemia, and rash, and the majority were grade 1&2 with no grade 4 toxicities. There was a significant reduction in serum VEGFR2 and PDGFR- on day-28 (p-values 0.0035 and 0.017, respectively). Both serum VEGF and sVEGFR-1 were increased on day-28, but the differences were not statistically significant (p-values 0.3223 and 0.084, respectively). Microarray analysis identified 32 suppressed genes with an FDR of <0.20 and at least a 2-fold change with no induced genes and 29 KEGG pathways were enriched on day-28. Our study suggests that sorafenib can restore endocrine sensitivity, particularly tamoxifen, and this strategy of adding novel agents in patients progressing on endocrine therapy should be examined in future trials.