Introduction: Clinical research on the predictive value of PIK3CA mutations in hormone receptor-positive (HR+), human epidermal growth factor 2-negative (HER2–) metastatic breast cancer (mBC) has advanced in recent years. However, knowledge of epidemiological prevalence has not been systematically evaluated. This study aimed to report prevalence of PIK3CA mutation using different biopsy techniques as well as specific hotspot mutations across the available literature. Methods: A comprehensive search of PubMed/MEDLINE, EMBASE, Cochrane Central, and select conference abstracts was performed by two independent researchers that included, but was not limited to, keywords: “breast neoplasm”, “PIK3CA protein”, “hormone receptor positive”, and “metastases”. English-language studies in HR+, HER2– mBC detailing the prevalence of PIK3CA mutations and published between January 1993 through August 2017 were included. Content analysis was employed to quantify collected data elements. Results: Of 558 studies included for full-text review, 36 met inclusion criteria. Most included studies (n = 18) were observational in nature. A total of 4,247 samples were tested for genetic mutations. Most studies used tissue biopsy samples (n = 33; 89%). Tumor samples accounted for 84.6% of all samples (n = 3597). Liquid biopsies were performed in 4 studies (11%) and accounted for 15.3% of all samples (n = 650). One study reported both liquid and tumor biopsy data. Overall, reported prevalence of the PIK3CA mutation ranged from 13.3% to 61.5%. Median prevalence was 36.4% (25th percentile = 28.6%; 75th percentile = 48.4%). Among studies using tissue biopsies, the majority reported prevalence from 16.7% to 61.5%. Among studies using liquid biopsies, the majority (n = 3) reported prevalence from 43.3% to 46.8%; one other study reported 13.3%. The most commonly tested hotspot mutations were H1047R and E545K. Among studies reporting specific hotspot mutation prevalence (n = 9), the H1047R hotspot mutation prevalence in these studies ranged between 25% and 75% while the E545K prevalence ranged between 11.1% and 50%. Conclusions: Although discrepancies exist with respect to mutation prevalence estimated across various tissue vs. liquid biopsy techniques, PIK3CA mutations and mutation hotspots (specifically H1047R and E545K) frequently occur in HR+/HER2– mBC. Citation Format: Lea Mollon, Alejandra Aguilar, Elizabeth Anderson, Joni Dean, Lisa Davis, Terri Warholak, Ayal A. Aizer, Emma Platt, Aditya Bardiya, Derek Tang. A systematic literature review of the prevalence of PIK3CA mutations and mutation hotspots in HR+/HER2- metastatic breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1207.