Leptin is a hormone which controls fat metabolism. Leptin plasma levels and adipose tissue mRNA expression were measured in cancer patients. Plasma levels were correlated with TNM staging, cachexia parameters, tumour markers and hormones. Breast and colorectal cancer patients showed blood plasma levels of insulin, TNF-alpha and tumour markers higher than controls. Breast cancer patients, but not colorectal cancer patients, had plasma levels and adipose tissue expression of leptin significantly higher than controls associated with elevated values of estrogen- and progesterone-receptors. These data suggest the possible use of leptin as a clinical marker.