Tumour necrosis factor (TNF) is a pleiotropic cytokine with activities that extend beyond its antitumour effect. There is now increasing evidence that TNF can be either constitutively produced or induced in human tumours. Tumour cells may also lead to TNF induction in normal cells. Experimental studies implicate TNF in processes that contribute to cancer progression. These range from stimulation of cancer growth and metastasis, to metabolic and haematological disturbances, e.g. cancer cachexia, anaemia, and hypercalcaemia. Future cancer therapies may therefore involve neutralisation of TNF activity in cancer patients.