Background: To examine the epidemiology and risk factors of chylothorax after cardiac procedure in the United States using a contemporary nationally representative database. Methods: We identified postoperative chylothorax events through National Inpatient Sample database (2016–2019) and compared baseline demographics, comorbidities, and in-hospital outcomes between hospitalizations with and without postoperative chylothorax. The Cochrane-Armitage test was used to analyze trends in incidence rates. Multivariable Poisson regression models were used to identify potential risk factors for postoperative chylothorax after cardiac procedure. Results: A total of 819 (0.24%) admissions were associated with postoperative chylothorax. The crude and standardized incidence rates of chylothorax were 23.7 (95%CI, 22.1–25.4) and 61.5 per 10,000 cardiac procedure-related admissions, respectively, with no significant temporal change in incidence rate over the study period (Ptrend = 0.5249). Infants [adjusted rate ratio (aRR), 117.3, 95% confidence interval (CI), 94.5–145.5] and children (aRR, 60.2, 95%CI, 48.0–75.5) were more likely to develop chylothorax compared to adults. Heart and great vessel procedures (aRR, 4.36, 95%CI, 3.61–5.26), septal repair (aRR, 1.91, 95%CI, 1.58–2.29), heart transplant (aRR, 5.68, 95%CI, 4.55–7.10) and pericardial procedures (aRR, 4.04, 95%CI, 3.32–4.91) were associated with elevated risk for chylothorax. Admissions with chylothorax were associated with higher inpatient mortality (4.9% vs. 3.0%, p