IMPORTANCE -  Recent data support percutaneous revascularization as an alternative to coronary artery bypass grafting in unprotected left main (ULM) coronary lesions. However, the relevance of these trials to current practice is unclear, as patterns and outcomes of ULM percutaneous coronary intervention (PCI) in contemporary US clinical practice are not well studied.

OBJECTIVE -  To define the current practice of ULM PCI and its outcomes and compare these with findings reported in clinical trials.

DESIGN, SETTING, AND PARTICIPANTS -  This cross-sectional multicenter analysis included data collected from 1662 institutions participating in the National Cardiovascular Data Registry (NCDR) CathPCI Registry between April 2009 and July 2016. Data were collected from 33 128 patients undergoing ULM PCI and 3 309 034 patients undergoing all other PCI. Data were analyzed from June 2017 to May 2018.

MAIN OUTCOMES AND MEASURES -  Patient and procedural characteristics and their temporal trends were compared between ULM PCI and all other PCI. In-hospital major adverse clinical events (ie, death, myocardial infarction, stroke, and emergent coronary artery bypass grafting) were compared using hierarchical logistic regression. Characteristics and outcomes were also compared against clinical trial cohorts.

RESULTS -  Of the 3 342 162 included patients, 2 223 570 (66.5%) were male, and the mean (SD) age was 64.2 (12.1) years. Unprotected left main PCI represented 1.0% (33 128 of 3 342 162) of all procedures, modestly increasing from 0.7% to 1.3% over time. The mean (SD) annualized ULM PCI volume was 0.5 (1.5) procedures for operators and 3.2 (6.1) procedures for facilities, with only 1808 of 10 971 operators (16.5%) and 892 of 1662 facilities (53.7%) performing an average of 1 or more ULM PCI annually. After adjustment, major adverse clinical events occurred more frequently with ULM PCI compared with all other PCI (odds ratio, 1.46; 95% CI, 1.39-1.53). Compared with clinical trial populations, patients in the CathPCI Registry were older with more comorbid conditions, and adverse events were more frequent.

CONCLUSIONS AND RELEVANCE -  Use of ULM PCI has increased over time, but overall use remains low. These findings suggest that ULM PCI occurs infrequently in the United States and in an older and more comorbid population than that seen in clinical trials.