Although common, the long-term significance of developing atrial fibrillation (AF) during a period of critical illness is unclear. We undertook a retrospective cohort analysis to assess the rate of thromboembolism (TE) in patients developing atrial fibrillation de novo during admission to our intensive care unit. In total, 1,955 patients were followed up (maximum follow-up 1,276 days) for the occurrence of TE, of which 220 (11.3%) had developed AF or atrial flutter during their critical care admission. There were 11 TE events among the patients with new AF (0.053 events per patient-year), compared with 18 in the non-AF group (0.0059 events per patient-year). The unadjusted hazard ratio for TE in patients developing new AF compared with those not developing AF was 8.09 (95% CI 3.08–17.19, p<0.001). In patients admitted to critical care, the development of AF appears to be associated with a significantly increased risk of subsequent thromboembolism.