Ectopic ACTH secretion due to malignant tumours is the most frequently underdiagnosed form of Cushing's syndrome. The majority of neoplasms causing ectopic ACTH syndrome are small-cell cancers of the lung or carcinoids. Other well-documented cases include adenocarcinoma of the lung, medullary thyroid carcinoma, pancreatic islet tumours and malignant thymoma. We report a rare case of metastatic colonic adenocarcinoma with ectopic ACTH syndrome. Clinical features such as proximal muscle weakness, peripheral oedema, hypertension or hirsutism in women, or the presence of unexplained hyperglycaemia, hypokalaemia or metabolic alkalosis in patients with known malignancy strongly suggest ectopic ACTH syndrome. Removal of the source of ACTH is the treatment of first choice, but often not feasible. Most often, treatment modalities are only palliative, with drugs directed against hypercortisolism such as aminoglutethimide, metyrapone, ketoconazole or mifepristone.