The authors report a review of 1808 patients admitted for the treatment of craniocerebral injuries. Eighty-four (4.6%) developed the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). Two types of SIADH (severe and mild) were defined on the basis of laboratory findings and clinical signs. SIADH occurred in 0.6% of the patients with mild head injury, 10.6% of those with moderate head injury, and 4.7% of the patients with severe head injury. Regular measurements of serum electrolytes, osmolality, and urinary sodium depletion should be made during the treatment of patients suffering from head trauma because unexpected clinical deterioration may often have a reversible cause: SIADH.