Muscle weakness is a common complaint in clinical practice. If this symptom is combined with focal liver lesions there is a broad spectrum of differential diagnoses for the gastroenterologist to consider. Tumors of neuroendocrine origin such as small-cell lung carcinoma (SCLC) produce a wide array of peptide hormones and are common causes of paraneoplastic syndromes. We report on a 68-year-old woman who presented with progressing muscle fatigue and multiple liver lesions on ultrasonography. Hypertension, hyperglycemia, hypokalemia and metabolic alkalosis prompted consideration of underlying hypercortisolism. Further work-up demonstrated an acute ectopic ACTH syndrome as paraneoplastic manifestation of a small cell lung carcinoma. The woman deteriorated rapidly and finally died from intracranial tumor spread and septic complications. This case stresses the diagnostic and therapeutic difficulties of acute ectopic ACTH syndrome in the setting of SCLC.