A spectrum of clinical manifestations is a feature of human lymphatic filariasis. The acute disease is characterized by periodic and self limiting episodes of adenolymphangitis, fever and associated constitutional symptoms, while the chronic disease includes long lasting manifestations such as lymphoedema and/or hydrocoele. The microfilariae carriers are generally free of clinical symptoms. In the present study circulating Tumour Necrosis Factor (TNF-alpha) was measured in human bancroftian filariasis with different clinical manifestations. Significantly elevated levels were observed only in patients with acute disease and not in microfilariae carriers or in patients with chronic manifestations. A detailed analysis of the acute cases indicated an absence of correlation between TNF-alpha levels and duration of the episodes. However, a significant positive correlation was observed between the severity of the disease and the TNF-alpha levels. About 85% of the acute cases with severe manifestations showed raised levels of TNF-alpha while only 6.5% of mild cases showed such levels. Manifestation of fever was also significantly associated with higher levels of TNF-alpha-while 80% of acute cases with fever had significant levels only 24% of acute cases without fever had high levels of TNF-alpha. Based on these observations we propose a mediatory role for TNF-alpha in acute filariasis and the possible use of TNF-alpha inhibitors for clinical management of the disease.