Hyponatremia is commonly seen in patients with severe and moderate head injury, but it is rarely reported in those with mild head injury. The authors report a patient with mild head injury who presented with data typical of inappropriate secretion of antidiuretic hormone (SIADH), but showed no clinical deterioration. Though the clinical significance of this condition is unclear, the true incidence of this pathology might well be found to be higher than expected, should it receive more clinical and/or serological attention. Continuing clinical assessment will be needed to determine the significance of this condition in relation to that in patients with SIADH following the various causes reported previously.