Paraneoplastic Cushing's syndrome caused by ectopic adrenocorticotropin production has been reported in association with a variety of malignant tumors. However, most cases of this syndrome are associated with neuroendocrine tumors such as small-cell lung cancer (SCLC), carcinoid tumors, and medullary carcinoma of the thyroid. We present a 49-year-old man who developed Cushing's syndrome associated with adenocarcinoma of the lung. He had a wide variety of clinical symptoms and comorbidity related to this syndrome during the course, and died 4 months after the diagnosis. This is a very rare case report of Cushing's syndrome associated with adenocarcinoma of the lung.