Acute and convalescent phase blood samples from five dengue fever (DF) patients and four dengue haemorrhagic fever (DHF) patients were tested for the presence of tumour necrosis factor (TNF). While all blood samples showed elevated levels, the acute phase blood sample levels were much higher. The mean TNF level in the acute samples of the five DF cases was 862 while in the DHF cases the level was 1722 pg/ml. Though the sample size is small, the difference appears to be statistically significant. Unlike in DF the distinctive features in DHF are the occurrence of shock, thrombocytopaenic purpura and sometimes disseminated intravascular coagulation (DIC). Increased TNF levels have not been reported in the literature in association with DHF, although it has been shown to contribute to these features which appear in some other diseases.