Recent studies have proven benefit of SGLT2i drugs in patients with heart failure with reduced ejection fraction (HFrEF), but their safety when combined with angiotensin-neprilysin inhibitor (ARNI) has not been established. The Safety and Efficacy of the Combination of Sacubitril/Valsartan and SGLT2i in HFrEF Patients registry was conducted to address this issue. SECSI registry is a consecutive, observational, retrospective, multicentre study conducted in 3 Heart Failure Units in Spain. It included 144 HFrEF patients who were treated with ARNI and iSGLT2. Data were collected at baseline, month 2, and month 6. The primary endpoint was the estimated glomerular filtration rate (eGFR), after the initiation of ARNI and sodium-glucose cotransporter-2 inhibitors (SGLT2i). Secondary endpoints included potassium levels and functional class (New York Heart Association class). There were 3 prespecified subgroup analyses: Elderly patients (≥70 years), patients with chronic kidney disease (KDIGO classification G3), and the sequence of drug initiation. Mean age was 69.9 ± 10.1 years, and 110 (76.4%) were men. Left ventricular ejection fraction was 32 ± 7.8%, and most patients were symptomatic 123 (87.2%) New York Heart Association II/III/IV. eGFR decreased at month 2 and this trend was maintained at month 6 eGFR baseline 68.5 ± 17.3, month 2 62 ± 19.7 and month 6 64.7 ± 8.6 mL/min/1.73 m2 (P < 0.01 for both). In prespecified analysis, elder patients and those who simultaneously initiate both treatments showed the steeper decrease in eGFR. To conclude, co-administration of SGLT2i and ARNI in routine care in HFrEF patients produced a slight decrease in eGFR at 6 months of follow-up. This decrease was especially significant in elder patients and those who initiate both drugs simultaneously.