Background: Aberrations of the PI3K/AKT/mTOR pathway are common in ER-positive breast cancer and may be associated with endocrine-resistance. We hypothesized that, since the majority of ER-positive breast cancers are actually sensitive to endocrine therapy, the most prevalent aberration in this pathway will more likely be associated with endocrine-sensitivity rather than resistance. Methods: We studied 28 tumors from patients enrolled on protocol NCT00570921 using fulvestrant and everolimus after aromatase inhibitor failure; 19 from 13 patients with endocrine-sensitive disease and 9 from 8 patients with resistant tumors. Resistance was defined as relapse within 3 years of adjuvant use or progression within 6 months in the metastatic setting. DNA was extracted from FFPE tumor tissue and subjected to next-generation-based comprehensive genomic profiling using the FoundationOne assay and alterations were then compared. Results: 10 of 13 patients with endocrine-sensitive tumors (77%) had at least one alteration in the PI3K/AKT/mTOR pathway vs. 6 of 8 patients with resistant disease (75%). PIK3CA mutations were more frequent in the sensitive group; 9/13 (69%) vs. 2/8 (25%) in the resistant group (p = 0.08). Interestingly, aberrations of pathway components other than PIK3CA were present in only 1 of 13 patients (8%) with sensitive disease vs. 4 of 8 patients (50%) with resistant disease (p = 0.0475). These aberrations were 1 activating AKT1 mutation in the sensitive group and 2 each of AKT1 activating mutations and PTEN loss in the resistant group. Further analysis of additional alterations will be presented in greater detail. Conclusions: Alterations in the PI3K/AKT/mTOR pathway are common in both endocrine-sensitive as well as resistant breast cancer but mutations of specific pathway components may distinguish sensitive from disease that is more likely to be resistant. PIK3CA mutations, which are relatively common, may be associated with more estrogen-dependent tumor biology while non-PIK3CA mutations are potentially associated with endocrine-resistance in ER-positive breast cancer. Further investigation of these findings is warranted.