The syndrome of inadequate secretion of the antidiuretic hormone (SIADH) is a very rare complication after carotid endarterectomy, characterized by hyponatremia, decrease of serum osmolarity as well as an increase in urinary osmolarity. We report the case of an 80-year-old woman who developed, 24 hours after the surgery, a picture of drowsiness and lethargy without neurological focality. The diagnosis of SIADH was suspected. We conclude that is important to have in mind this clinical entity in the differential diagnosis of non-focal neurological deficit after carotid endarterectomy.