Hyponatremia in association with a lung mass usually indicates small cell lung cancer (SCLC) with inappropriate antidiuretic hormone (ADH) production by the tumor. About 10% of lung cancers present with a paraneoplastic syndrome. Tumors producing ADH or adrenocorticotropic hormone (ACTH) are overwhelmingly SCLCs, which arise from hormonally active neuroendocrine cells. SCLC is a rapidly growing neoplasm; early mediastinal involvement, as in this case, is common. Tumor staging for SCLC differs from non-small cell cancers. SCLCs are simply classified as limited (confined to one hemithorax) or extensive. Limited tumors are usually managed with combination radiation and chemotherapy, with approximately 20% cure rate. Extensive tumors are treated with palliative chemotherapy alone; durable remissions are rare. Surgery is not curative in SCLC.Bronchial carcinoids are usually benign. Although they can produce ACTH, mediastinal involvement and hyponatremia would not be expected. Adenocarcinoma of the lung, although common, rarely causes a paraneoplastic syndrome. Localized benign lung infections (especially lung abscess) can cause syndrome of inappropriate antidiuretic hormone (SIADH) but would not account for this patient's mediastinal adenopathy. Lung abscess usually causes fever and fetid sputum. Pulmonary aspergilloma (a fungus ball growing in an old cavitary lesion) can cause hemoptysis but not this patient's hyponatremia or mediastinal lymphadenopathy.