We describe a patient who had metastatic carcinoma of the stomach and severe headaches, weakness, and diplopia. The patient had meningeal carcinomatosis that was confirmed by examination of the cerebrospinal fluid after computed tomography of the brain was normal. Weakness was explained by the finding of hyponatremia due to the syndrome of inappropriate antidiuretic hormone (SIADH). We believe this is the first reported case of meningeal carcinomatosis in which stomach carcinoma caused SIADH. The differentiation between SIADH and cerebral salt wasting due to malignant tumors is emphasized.