Cushing's syndrome due to young small-cell lung cancer (SCLC) is recognized as being extremely rare. We herein present the case of a 35-year-old nonsmoking man who presented with thirst and polyuria. Laboratory examinations showed hyperglycemia, hypokalemia and liver enzyme elevation. Imaging examinations revealed the presence of multiple liver tumors and lymph node swelling. The levels of serum neuroendocrine tumor markers were elevated. The patient was diagnosed with SCLC based on the pathological examination of a biopsy specimen from the right supraclavicular lymph node. The physical findings, including proximal myopathy, truncal obesity and pigmentation suggested high levels of glucocorticoids. An immunohistochemical examination of the tumor showed that it was positive for adrenocorticotropin (ACTH). An endocrinological investigation allowed for the definitive diagnosis of SCLC with ectopic ACTH production.