Ectopic ACTH syndrome is a rare cause of Cushing's syndrome accounting for about 15% of all cases. Small cell lung cancer and bronchial carcinoids account for about half of the cases. Malignant neoplasm has rapid and more aggressive metabolic effects. We report a 54-year-old male patient with phenotypic features of Cushing's syndrome with severe hypokalemia, metabolic alkalosis, hypertension and altered mental status as manifestations of an ACTH-secreting small cell carcinoma from the lung. Ectopic ACTH syndrome should be highly considered in patients with hypertension and severe hypokalemic metabolic alkalosis, especially when a lung mass is discovered.