Experience with angiotensin-receptor neprilysin inhibitors (ARNI) in oncologic patients with heart failure (HF) is limited. We report a case of ARNI started as first-choice therapy in a patient with relapsing hairy cell leukaemia (HCL) and HF with depressed left ventricular ejection fraction (LVEF). A middle-aged male, previously treated with rituximab for HCL, was scheduled for cardiologic screening before starting a new antineoplastic therapy for cancer relapse. The patient had symptomatic HF with reduced LVEF and high NT-proBNP levels. In this patient, early ARNI treatment was well tolerated and produced a rapid and durable improvement of symptoms, LVEF and NT-proBNP levels. Consequently, the oncologic team could start an experimental treatment with obinutuzumab, with complete HCL remission. In conclusion, in this patient with HCL and HF, ARNI therapy was safe and effective, contributing to undelayed cancer treatment.