Serial estimations of serum cortisol were performed in 49 patients with craniocerebral trauma. Abnormalities of serum cortisol, including alterations in diurnal rhythm and elevations of serum cortisol level, occurred in 21 patients. The frequency and severity of the abnormalities correlated with the severity of the head injury, and there was a trend suggesting that middle fossa basal skull fractures predisposed to cortisol abnormalities. A further 6 patients were studied to assess the effects of exogenous dexamethasone, and in all patients there was suppression of elevated serum cortisol levels by the dexamethasone. The findings suggest that hypercortisolemia after head injury is related to an alteration rather than an abolition of the normal feedback mechanism.