Background: Intravascular ultrasound (IVUS) guides deferral decision-making regarding the left main coronary artery (LMCA) and improves outcomes. Further studies regarding coronary physiology to guide revascularization in the LMCA are needed. Our aim was to evaluate the outcome of LMCA deferral using IVUS or coronary physiology via instantaneous wave-free ratio (iFR) or fractional flow reserve (FFR). Methods: Between January 2014 and February 2022, patients undergoing evaluation with either IVUS or coronary physiology in the LMCA were included from the SWEDEHEART registry. Exclusion criteria were a minimum luminal area < 6 mm2, iFR < 0.89, FFR < 0.80, ad hoc percutaneous coronary intervention of lesions in the LMCA, proximal left anterior descending artery, and proximal circumflex artery, planned elective revascularization, and planned valvular surgery. The primary outcome was major adverse cardiac events (MACE), defined as a composite of all-cause death, myocardial infarction, and unplanned revascularization. Kaplan-Meier event rates and multivariable Poisson regression were used for the statistical analyses. Results: Deferral of revascularization in the LMCA was performed in 1552 patients, 33.6 % with IVUS and 66.4 % with coronary physiology (iFR 11.3 % vs. FFR 55.0 %). The median follow-up time was 2.7 years. No significant difference was seen in MACE (IVUS 40.2 % vs. coronary physiology 35.5 %; adjusted RR: 1.18; 95 %CI: 0.97-1.44; p = 0.09). The results were consistent across all investigated subgroups. The rate of all-cause death was higher in the IVUS group (adjusted RR: 1.38; 95 %CI: 1.03-1.83; p = 0.03). Conclusions: Deferral of coronary revascularization in LMCA lesions using IVUS or coronary physiology did not differ in our combined endpoint. We observed a higher risk of all-cause death using IVUS.