OBJECTIVES:  Targeted therapy attacks specific types of cancer cells. Companion diagnostics (CDx) aid in the identification of candidates who may benefit from a targeted therapy. This analysis was conducted to understand the trend of targeted therapy/CDx utilization under Medicare, 2011-2015. METHODS:  Therapies were selected based on FDA cleared/approved CDx. We analyzed Medicare Part D drug spending (2011-2015) published by CMS in 2016 for each therapy. Analysis was limited to therapies with a single cancer indication requiring CDx. Further analysis considering all targeted therapies with CDx were conducted to understand impacts of the therapy exclusion criteria. RESULTS: From 2011 to 2015, the number of targeted cancer therapy beneficiaries increased from 498 to 7,133. The Medicare spend increased from $9,547,737 to $293,607,643, also increasing as part of the overall Medicare Part D spend (0.01% to 0.21%). PMPM increased from $0.0163 to $0.4408. In 2011, the total spend as part of the overall spend on these included therapies was led by melanoma (61.24%), followed by non-small cell lung cancer (NSCLC) (33.76%) and colorectal cancer (CRC) (5.00%). This trend has changed and by 2015, there was increased utilization of targeted therapies for NSCLC (51.58%), followed by melanoma (36.32%), ovarian cancer (9.65%), breast cancer (2.03%) and CRC (0.42%). In 2015, the annual spend per beneficiary ranged from $15,919 (Gefitinib) to $70,967 (Crizotinib), and the cost sharing was 0.0045-0.5110% for beneficiaries who qualified for a Low Income Subsidy (LIS), and 5.1%-17.1% for those who do not qualify for LIS. Further analysis demonstrated a similar trend in the therapy utilization. CONCLUSIONS: The Medicare spend on targeted cancer therapies requiring CDx increased 2011-2015, and also increased as part of the overall Medicare Part D spend. It is expected that this trend may continue, driven by the increasing adoption of targeted therapies in clinical practice, and the target therapies currently under development.