Background/Introduction: The correlation between low body mass index (BMI) and congestive heart failure (obesity paradox) has been described in the literature; however, the association between BMI and clinical outcome measures is not well characterized. Purpose: This study aimed to describe the correlation among BMI and myocardial infarction (MI)/cerebrovascular accident (CVA)/mortality composite outcome, combined infection/bleeding and combined CVA/transient ischaemic attacks (TIA)/unexplained syncope outcome measures of patients with heart failure with reduced ejection fraction (HFrEF). Methods: Retrospective analysis of prospectively collected data of a single centre heart failure registry in Saudi Arabia. Results: Of 167 patients with HFrEF, 41 (25%) had BMI of ≤24 kg/m2. The low BMI group had higher composite MI/CVA/mortality (19% vs 3%; P = 0.014) and composite infection/bleeding (12% vs 19%; P = 0.016) rates compared with those with BMI of>24 kg/m2. Moreover, the study revealed that lower the BMI, higher the CVA/TIA/unexplained syncope (23.9, 5.5 vs 28.7, 6.2; P = 0.01), higher the composite MI/CVA/mortality (26.4, 5.6 vs 29, 6.3; P = 0.014) and lower the combined infection/bleeding (27.7, 5.8 vs 31.3, 7.3; P = 0.01) rates. Conclusion: Patients with HFrEF having BMI of ≤24 kg/m2 had a higher incidence of MI/CVA/mortality, higher combined CVA/TIA/unexplained syncope incidence and lower combined incidence of bleeding/infection compared with those having BMI of>24 kg/m2.