Measurement of disease activity in patients with inflammatory bowel disease is difficult. The best available methods are complex and time consuming, but it may be possible to use tumour necrosis factor alpha (TNF alpha) concentration in stool as a marker of disease activity. We measured TNF alpha concentrations in stool samples from normal children, infants with diarrhoea, and children with inflammatory bowel disease in active and inactive phases of the disease. In 10 normal children and 14 children with diarrhoea, median stool TNF alpha concentrations were 58 and 45 pg/g stool, respectively. Compared with diarrhoeal controls, stool TNF alpha concentrations were significantly increased in children with active Crohn's disease (n = 13, median 994 pg/g, p less than 0.0002) and active ulcerative colitis (n = 4, range 276-5982 pg/g, p less than 0.003). In patients with inactive disease, either as a result of surgery or treatment with steroids, the concentration of stool TNF alpha fell to those of controls. Measurement of stool TNF alpha concentrations may provide a simple way to monitor disease activity in inflammatory bowel disease.