OBJECTIVE To examine the effectiveness of low-dose lithium carbonate for managing carbamazepine-induced hyponatremia. METHODS Single case study in an 88 year old man with bipolar illness and vascular dementia who had failed to respond to other mood stabilizers. RESULTS The patient had developed hyponatremia on two separate occasions when treated with carbamazepine. Introduction of low-dose lithium resulted in prompt normalization of serum sodium levels, which was maintained for the subsequent 8 weeks. CONCLUSIONS Carbamazepine may sometimes be the best or only viable treatment option for patients with bipolar illness or other conditions. When its use is complicated by syndrome of inappropriate ADH, dose reduction and fluid restriction are the simplest options but, if ineffective, addition of lithium may be a feasible, albeit somewhat complicated, alternative.