Introduction: Heart failure is present in one out of three patients on hemodialysis and is the leading cause of death. Because increased blood flow through the arteriovenous fistula (AVF) leads to volume overload potentially causing high-output heart failure, reduction (e.g. aneurysmorrhaphy) or occlusion of a hyperfunctional AVF (hAVF) is commonly performed. However, there are no clear indications for these procedures. The aim of this study was to evaluate the impact of aneurysmorrhaphy on hemodynamics and cardiac structure in patients with hAVF. Methods: Thirty patients (age 52 ± 12 years, 73% male) with hAVF (flow ≥ 1.5 L/min) were examined before, at 6 and 12 months after aneurysmorrhaphy. Results: Relationship between AVF flow and cardiac index (CI) was weak and nonlinear. In total, 16 patients had normal CI (2.5 - 3.8 L/min/m2) and 14 high CI (4.0 - 6.0 L/min/m2). While the AVF flow did not differ between groups (3.1±1.4 vs. 3.5±0.8 L/min, p=0.34), there was a difference in CI (3.3±0.7 vs. 4.7±0.5, p<0.001). After operation AVF flow significantly and similarly decreased in both groups, while CI decreased only in the high CI group (Table 1). Similarly, there was a significant decrease in left ventricular end-diastolic diameter and mass, left atrial and right ventricular diameter and pulmonary systolic pressure in the high CI, but not in the normal CI group (Table 1). Conclusions: Aneurysmorrhaphy of a hAVF leads to positive heart remodeling only in patients with elevated CI. Increased CI, but not increased AVF flow, should be used as an indication for aneurysmorrhaphy.