The pathogenesis of dengue haemorrhagic fever (DHF) is not well understood. In the absence of predictive clinical or biological criteria, the management of DHF patients remains difficult. The role played by cytokines in the occurrence of DHF has been suggested by several authors. In this study, we determined the plasma levels of tumour necrosis factor alpha (TNF alpha) and transforming growth factor beta-1 (TGF beta-1) in 52 children with laboratory-confirmed dengue virus infection admitted to hospital during the recent dengue 2 outbreak in French Polynesia. Thirty-three children were classified as having dengue fever (DF) and 19 as DHF. The plasma of both DF and DHF patients contained similar levels of TNF alpha. By contrast, plasma obtained from children with DHF had significantly higher levels of TGF beta-1 than plasma from children with DF, especially from days 1 to 3 after the onset of fever.