AIMS: Atrial fibrillation or flutter (AFl) frequently coexists with chronic heart failure (CHF), yet its impact on in-hospital outcomes in large, unselected populations remains insufficiently defined. METHODS: We analyzed all hospitalizations for CHF (ICD-10-GM I50*) in Germany from 2014 to 2022 using nationwide administrative data. Patients were stratified by rhythm status (AFl vs. non-AFl). Outcomes included in-hospital mortality, complications, length of stay, and health care costs. Multivariable logistic regression was used to identify independent predictors of mortality. RESULTS: Among 4,057,291 hospitalizations, 56.9% had AFl. These patients were older (median 82 vs. 79 years), more often female, and showed a higher comorbidity burden, particularly CKD, hypertension, and stroke. AFl was associated with higher rates of complications such as AKI (11.9% vs. 9.7%; p < 0.001) and cardiogenic shock (1.15% vs. 1.03%; p < 0.001), but lower use of mechanical ventilation and assist devices. Despite this, in-hospital mortality was slightly lower in AFl (8.30% vs. 8.66%). AFl patients had longer median hospital stays (8 vs. 7 days), while costs were comparable. Ventilated AFl patients had longer ventilation times (28 vs. 21 hours). In multivariable analysis, AFl was independently associated with higher mortality in most age groups, except for patients aged 40-49, 80-89, and > 90 years. CONCLUSIONS: In this nationwide cohort, AFl was common among CHF patients and linked to more advanced disease. Its impact on in-hospital mortality was age-dependent and complex, highlighting the need for tailored clinical management.