Interleukin (IL)-13 is produced by T helper 2 (Th2)-type cells and inhibits the production of proinflammatory cytokines by activated monocytes, while IL-18 is a pleiotropic cytokine that induces interferon-γ and plays an important role in the development of Th1-type cells. Role of the shift from a Th1-type response to Th2-type has been suggested in the pathogenesis of dengue hemorrhagic fever (DHF). This study was undertaken to investigate the possible protective/pathogenic role of IL-13 and IL-18 in patients with DHF. Sera were collected from a total of 84 patients with various grades of dengue illness and 21 normal healthy controls and tested for IL-13 and IL-18 levels using commercial enzyme-linked immunosorbent assay kits. The results showed that very low levels of IL-13 (4±3 pg ml⁻¹) and IL-18 (15±4 pg ml⁻¹) were detected in the sera of healthy controls. In dengue patients, the levels of IL-13 and IL-18 were the highest in the patients with DHF grade IV (205±103 pg ml⁻¹ and 366±155 pg ml⁻¹, respectively) and the lowest in patients with dengue fever (22±12 pg ml⁻¹ and 76±50 pg ml⁻¹, respectively). Both the cytokines appeared (IL-13=20±11 pg ml⁻¹ and IL-18=70±45 pg ml⁻¹) during the first 4 days of illness and reached peak levels (IL-13=204±96 pg ml⁻¹ and IL-18=360±148 pg ml⁻¹) by day 9 onwards. The presence of high levels of IL-13 and IL-18 during severe illness and late phases of the disease suggests that both of these cytokines may contribute to the shift from a Th1- to Th2-type response and thus to the pathogenesis of DHF.