OBJECTIVES: Low molecular weight heparin (LMWH) may provide beneficial effects on outcomes of COVID-19. We aimed to examine the impact of LMWH treatment on clinical outcomes (duration of hospitalization, admission to intensive care unit, the requirement for mechanical ventilation, and death) of COVID-19 patients with normal D-dimer levels at admission. BACKGROUND: Coronavirus disease-2019 (COVID-19) predisposes patients to arterial and venous thrombosis. METHODS: In this retrospective, multicentre and observational study we analysed the data of 308 confirmed COVID-19 patients with normal D-dimer levels at initial admission. After propensity score matching (PSM) patients were grouped; Group 1; patients who received LMWH with D-dimer </=0.5 mg/L, Group 2; patients who received LMWH after D-dimer levels exceeded 0.5 mg/L, and Group 3; patients who did not receive LMWH. RESULTS: After PSM, each group comprised 40 patients. The patients in Group1 had the best clinical outcomes compared to the other groups. Group 3 had the worst clinical outcomes (p<0.005). The benefit of LMWH increased with early prophylactic therapy especially when started while the D-dimer levels were </=0.5 mg/L. CONCLUSION: Our results strongly suggest that proactive LMWH therapy improves clinical outcomes in hospitalized COVID-19 patients even with normal D-dimer levels (</= 0.5 mg/L) (Tab. 3, Fig. 2, Ref. 34).