AIMS: Amongst patients with critical illness associated new onset AF (CI-NOAF), the risk of subsequent atrial fibrillation (AF) diagnoses and other adverse outcomes is unknown, and the role for long-term anticoagulation is unclear. This study sought to determine the factors associated with subsequent AF diagnoses and other adverse outcomes in this cohort. METHODS AND RESULTS: Admissions to a tertiary general intensive care unit (ICU) between December 2015 and September 2018 were screened for AF episodes through hourly analysis of continuous ECG monitoring. Patients with a prior history of AF were excluded. AF burden was defined as the percentage of monitored ICU hours in AF. The primary endpoint was subsequent AF diagnoses, as collated from the statewide electronic medical records. Secondary endpoints included mortality, embolic events, MACE and subsequent anticoagulation. RESULTS: Of 7030 admissions with 509 303 h of monitoring data, 309 patients with CI-NOAF were identified, and 235 survived to discharge. Subsequent AF diagnoses were identified in 75 (31.9%) patients after a median of 413 days. Increased AF burden had the strongest independent association with AF recurrence (OR = 15.03, P = 0.002), followed by increased left atrial area (OR = 1.12, P = 0.01). Only 128 (54.5%) patients had their AF diagnosis acknowledged at ICU discharge, and 50 (21.3%) received anticoagulation at hospital discharge. CONCLUSION: CI-NOAF is often under-recognized, and subsequent AF diagnoses are common post-discharge. AF burden during ICU admission has a strong independent association with subsequent AF diagnoses. Left atrial size is also independently associated with subsequent AF.