Background: Aromatase inhibitor (AI) is standard of care for patients with hormone receptor positive (HR⁺) human epidermal growth factor receptor 2-negative (HER2^-) metastatic breast cancer (MBC). The current phase I trial was designed to test the safety and efficacy of AI and the immune checkpoint inhibitor pembrolizumab (NCT 02648477). Methods: Key eligibility criteria were HR⁺ HER2^- MBC per ASCO/CAP; RECIST 1.1 measurable disease; adequate organ function; and ECOG 0-1. Eligible patients received 200 mg pembrolizumab IV every 3 weeks plus AI until progression or unacceptable toxicity. Primary objectives were to evaluate the safety and efficacy of this combination. This study employed a 3-at-risk design with a lead-in at the standard dosing of both AI and pembrolizumab with a targeted accrual of 20 patients. Results: A total of 20 patients were accrued between March 2016 and April 2017. Median age was 62 (range 34-79), with 75% white, 15% Asian and 10% unknown. Median lines of therapy were 3 (0, 9). All but one patient received aromatase inhibitor and/or fulvestrant prior to enrollment. The combination was well tolerated, and the most common adverse events were grade 2 fatigue (35%), rash (15%), and hot flashes (10%). Grade 3 adverse events were elevated AST/ALT (5%), rash (5%), and lymphopenia (5%). Responses were 10% partial response and 15% stable disease, resulting in a clinical benefit rate (CBR) of 20% at 6 months. Median follow-up time was 40.1 months (range 31.3 – 46.8 months). Median progression free survival was 1.8 months (95% CI 1.6, 2.6) and median overall survival was 17.2 months (95% CI 9.4, NA). 14 tumor specimens had programmed death ligand 1–positive (PD-L1) by 22C3 testing, including 3 PD-L1-positive and 11 PD-L1 negative. No association between PD-L1 and response was found. Conclusions: The combination of pembrolizumab and AI is well tolerated in patients with HR⁺ HER2^- MBC who were not pre-selected for PD-L1. There was minimal overall clinical activity observed beyond what was to be expected with AI alone in this group of patients. Clinical trial information: NCT 02648477.