Purpose: Current guidelines recommend the use of low molecular weight heparin (LMWH) for most haemodynamically stable patients with pulmonary thromboembolism (PTE), however, it is not clear whether LMWH is preferable to unfractionated heparin (UFH) for the treatment of massive PTE. We aimed to compare the use of LMWH versus UFH after thrombolytic treatment in the management of acute massive PTE for hemorrhage and hospital mortality.Methods: The study, a randomized, single center, parallel design trial, included the patients who had confirmed the diagnosis of massive PTE according to clinical findings and computerized thorax angiography and no contraindication to the treatment between January 2011 and October 2013. After thrombolytic treatment, the patients assigned to therapy with LMWH or UFH. Any hemorrhage, major hemorrhage, and hospital mortality were assessed.Results: A total of 121 patients, 71 female (58.7 %) and 50 male (41.3 %), who had massive PTE with an average age 62.6 ± 15.7 (ranges 22-87) were included for analyses in the study. They were allocated to either LMWH (n = 60) or UFH (n = 61) group. Although the occurrence of any adverse event (21.7 vs 27.9 %) and each individual type of adverse event were all lower in the LMWH group compared to UFH group (6.7 vs 11.5 %, 3.3 vs 9.8 %, and 15.0 vs 19.7 % for death, major hemorrhage, and any hemorrhage, respectively), the differences were not statistically significant.Conclusions: Our findings suggest that LMWH might be a better option in the management of the patients with massive PTE. Multi-center larger randomized controlled trials are required to confirm our results.