A significant percentage of dengue patients develop Dengue Shock Syndrome (DSS) which is characterized by increased vascular permeability, circulatory failure and often death. Montelukast, a cysteinyl leukotriene receptor antagonist regulates vascular permeability and we hypothesized that it may be effective in protecting against DSS. An open label, parallel, randomized controlled trial (RCT) was thus carried out at Mayo Hospital, Department of Medicine, Lahore. A total of 200 patients of dengue fever were recruited and randomized into two groups. The group A was treated with Montelukast 10 mg once daily for 5 days along with general supportive treatment. Group B received the standard supportive treatment and served as the control group. The frequency of DSS was compared in the two groups by Chi square test. A binary logistic regression analysis was conducted to assess the effects of montelukast treatment on onset of DSS after adjusting for gender, age, white cell count, platelet count, haematocrit, serum alanine transaminase (ALT) and aspartate transaminase (AST). Relative risk (RR), absolute risk reduction (ARR), relative risk reduction (RRR) and numbers needed to treat (NNT) were calculated. Significance level was set at p<0.05. We found that only 9% of the patients in treatment group developed DSS compared to 31% patients in group B (p<0.001). The protective effect of montelukast treatment persisted (p>0.001, Odds ratio=5.01, 95% CI=2.17-11.60) even after adjusting for confounders. Montelukast reduced the absolute risk (ARR=22%) and the relative risk (RRR=71%) of DSS in dengue fever. Numbers needed to treat were 4.55. We thus conclude that treatment with oral montelukast may protect patients of dengue fever from DSS and greatly reduce mortality.