To assess the involvement of inflammatory mediators in the development of adverse reactions in filarial patients undergoing treatment, 29 microfilaremic subjects were treated with diethylcarbamazine (DEC). Before and at serial time points after initiation of treatment, plasma levels of inflammatory mediators and DEC were measured, and adverse reactions were recorded. Patients experienced no or mild, moderate, or severe adverse reactions. Increasing pretreatment microfilarial counts were associated with escalating severity of adverse reactions. Plasma concentrations of DEC were not different among patients suffering from varying degrees of illness. Interleukin (IL)-6, IL-10, lipopolysaccharide-binding protein (LBP), and soluble tumor necrosis factor receptors (sTNF-Rs) increased after treatment. IL-6 and LBP, however, showed the strongest association with adverse reactions. Increasing levels of these molecules were closely correlated with the mounting severity of adverse reactions, which raises the possibility that they play an important role in systemic inflammation that arises after DEC treatment of filarial patients.