Posttraumatic diabetes insipidus has been reported as a sequela to head injury. It is unknown whether subclinical types of diabetes insipidus, or other types of water metabolism disorders, occur after mild head injury (MHI) and, if so, whether they are related to the persistence of postconcussional symptoms. MHI patients (n = 38) were screened for disturbances of water metabolism by comparing plasma and urine osmolalities at about 5 weeks after the trauma. Eight patients had evidence of an increased plasma osmolality together with a relatively decreased urine osmolality after an overnight fast. The presence of this disturbance was significantly related to the persistence of postconcussional symptoms. The results suggest that subclinical disturbances of water metabolism may, among other factors, be related to the persistence of symptoms after MHI.