The role of immunological factors in the pathogenesis of Dengue Hemorrhagic Fever (DHF), particularly Dengue Shock Syndrome (DSS) was studied. The complement activity, circulating immune complex (IC), histamine level, platelets and leucocytes were determined in the blood of 30 patients and 43 healthy persons. The results showed a significant decrease in complement activity, platelet and neutrophil counts, an increase of histamine level (P less than 0.001), and the presence of circulating IC in 80 per cent of our patients. Furthermore, a marked correlation was found between the changes of the above parameters and the clinical stages of the disease. These parameters can be used in the prognostics of the pre-shock and shock syndrome of DHF.