Abstract Objectives The authors evaluated mortality and indices of cost of care among inpatients with Atrial Fibrillation (AF) and a diagnosis of a Temperature-Related Illness (TRI). The authors also assessed trends in the prevalence of TRIs among AF hospitalizations. Methods In this cross-sectional study, the authors used discharge data from the Nationwide Inpatient Sample (NIS) collected between January 2005 and September 2015 to identify patients with a diagnosis of AF and TRI. Outcomes of interest included in-hospital mortality, invasive mechanical ventilation, hospital length of stay, and cost of hospitalization. Results A total of 37,933 encounters were included. The median age was 79 years. Males were slightly overrepresented relative to females (54.2% vs. 45.8%, respectively). Although Blacks were only 6.6% of the cohort, they represented 12.2% of the TRI cases. Compared to non-TRI-related hospitalizations, a diagnosis of a TRI was associated with an increased likelihood of invasive mechanical ventilation (16.5% vs. 4.1%, p< 0.001), longer length-of-stay (5 vs. 4 days, p <0.001), higher cost of care (10,082 vs. 8,607, in US dollars p <0.001), and increased mortality (18.6% vs. 5.1%, p <0.001). Compared to non-TRI, cold-related illness portends higher odds of mortality 4.68, 95% Confidence Interval (4.35-5.04), p <0.001, and heat-related illness was associated with less odds of mortality, but this was not statistically significant 0.77 (0.57-1.03), p= 0.88. Conclusion The occurrence of TRI among hospitalized AF patients is small but there is an increasing trend in the prevalence, which more than doubled over the decade in this study. Individuals with AF who are admitted with a TRI face significantly poorer outcomes than those admitted without a TRI including higher mortality. Cold-related illness is associated with higher odds of mortality. Further research is required to elucidate the pathogenic mechanisms underlying these findings and identify strategies to prevent TRIs in AF patients. (AF TemperatureRelated Temperature Related . (TRI) hospitalizations crosssectional cross sectional study NIS (NIS 200 201 inhospital stay hospitalization 37933 37 933 37,93 7 years 54.2% 542 54 2 (54.2 vs 458 45 8 45.8% respectively. respectively respectively) 66 6 6.6 cohort 122 12 12.2 cases nonTRIrelated non related 16.5% 165 16 5 (16.5 41 1 4.1% 0.001, 0001 0.001 , 0 001 0.001) lengthofstay ( days <0.001 <0.001) 10,082 10082 10 082 (10,08 8607 607 8,607 18.6% 186 18 (18.6 51 5.1% <0.001. nonTRI, nonTRI TRI, non-TRI coldrelated cold 468 68 4.68 95 4.355.04, 435504 4.35 5.04 35 04 (4.35-5.04) heatrelated heat 077 77 0.7 0.571.03, 057103 0.57 1.03 57 03 (0.57-1.03) 088 88 0.88 Coldrelated Cold (TRI 20 3793 3 93 37,9 54.2 (54. 45.8 6. 12. 16.5 (16. 4.1 000 0.00 00 <0.00 10,08 1008 08 (10,0 860 60 8,60 18.6 (18. 5.1 46 4.6 9 355 4.355.04 43550 435 4.3 504 5.0 (4.35-5.04 07 0. 571 0.571.03 05710 057 0.5 103 1.0 (0.57-1.03 0.8 379 37, 54. (54 45. 16. (16 4. 0.0 <0.0 10,0 100 (10, 86 8,6 18. (18 5. 4.355.0 4355 43 50 (4.35-5.0 0.571.0 0571 05 1. (0.57-1.0 (1 <0. 10, (10 8, 4.355. (4.35-5. 0.571. (0.57-1. <0 4.355 (4.35-5 0.571 (0.57-1 < (4.35- (0.57- (4.35 (0.57 (4.3 (0.5 (4. (0. (4 (0