Study objective: The objective was to assess the effect of ongoing angiotensin receptor-neprilysin inhibitor(ARNI) on the effect of the sodium glucose cotransporter 2 (SGLT2) inhibitor empagliflozin on left ventricular (LV) size and function in patients with heart failure and reduced ejection fraction(HFrEF). Design: Post hoc analysis of the Empire HF trial, an investigator-initiated, double-blind, randomized controlled trial. Participants: 190 patients with HFrEF with New York Heart association class I-III symptoms with an ejection fraction of 40 % or below. Patients were stratified according to ongoing ARNI treatment at baseline. Intervention: Empagliflozin 10 mg daily or placebo for 12 weeks. Echocardiography at baseline and follow-up. Main outcome measures: Left ventricular end-systolic volume index (LVESVI), end-diastolic volume index (LVEDVI), left atrial volume index (LAVI), left ventricular ejection fraction (LVEF). Results: A total of 58 patients (31 %) received ARNI at baseline. Compared to with placebo, empagliflozin reduced the LVESVI ([−6.2 (−14.1 to 1.6); p = 0.12] and [−3.3 (−8.2 to 1.6); p = 0.19], interaction P = 0.49), LVEDVI ([−11.2 (−21.2 to −1.2); p = 0.03] and [−2.9 (−8.7 to 2.9); p = 0.32], interaction P = 0.13), and LAVI ([−3.9 (−9.1 to 1.2); p = 0.14] and. [−1.8 (−4.4 to 0.7); p = 0.16], respectively, interaction P = 0.9) in patients treated with and without ARNI at baseline, respectively. No treatment-by-ARNI subgroup interaction were found. Unaffected by baseline ARNI treatment, empagliflozin did not improve LVEF. Conclusion: The effect of empagliflozin on cardiac structure and function compared to placebo was not affected by background treatment with ARNI.