Water diuresis after head trauma is most often due to central diabetes insipidus (DI). We report a patient with a history of a bipolar disorder and past lithium use who was noted to have polyuria and hypernatremia after head trauma. Inappropriate high sodium and volume replacement resulted in an increase in the polyuria. A lack of response to antidiuretic hormone/antidiuretic-hormone-like preparations led to the diagnosis of nephrogenic DI. The case illustrates the importance of calculating electrolyte-free osmolar clearance in the correction of hypernatremia. Persistence of the DI and mild renal impairment probably due to past lithium use are discussed.