Fasting blood samples were collected from 83 patients with histologically proven breast cancer and analysed for plasma glucagon, serum immunoreactive tumour necrosis factor (TNF alpha), insulin, glucose, growth hormone, cortisol and TSH. Samples from patients with known diabetes mellitus or thyroid disease, and those on parenteral nutrition or with evidence of infection were excluded as were patients who had a history of weight loss through dieting or who were anorexic. Fasting plasma glucagon, serum cortisol and immunoreactive TNF alpha concentrations in patients with stage IV breast cancer who had developed weight loss were significantly higher than those in patients with stage IV disease who had not developed weight loss. There were no significant differences in the fasting serum concentrations of insulin, glucose, growth hormone and TSH between the two patient groups. The association between weight loss in stage IV breast cancer and increased concentrations of plasma glucagon, serum cortisol and TNF alpha suggests a possible role for these hormonal factors in the development of cancer cachexia.