152 ©  Blackwell Science Ltd, Journal of Intellectual Disability Research , – Abstract Carbamazepine is an anticonvulsant and psy- chotropic medication commonly used in the treat- ment of people with intellectual disability (ID). The incidence of hyponatremia during treatment in this population is unclear. The present study aimed to determine the prevalence of hyponatremia during carbamazepine treatment in patients with ID, and to investigate the risk factors and clinical features of this condition. The prevalence of hyponatremia was retrospectively assessed in  people receiving carbamazepine (subject group) and  people not receiving carbamazepine (control group) who lived in a residential centre for people with ID. The relationship between serum sodium level, sex, age, daily carbamazepine dose and serum carba- mazepine levels was examined. The prevalence of the clinical features of hyponatremia was assessed in this population using a checklist. The prevalence of hyponatremia was .% and .% in the subject and control groups, respectively. The mean serum sodium level in the subject group was significantly lower than that in the control group. Hyponatremia correlates significantly with a high daily carbamazepine dose and a high serum carbamazepine level. The checklist of clinical features was not useful in detecting hyponatremia clinically. Hyponatremia is a common occurrence in this population. In the light of the uncertain sig- nificance of mild, chronic hyponatremia, the value of routine monitoring of serum electrolytes has yet to be established. Keywords adverse effects, carbamazepine, hypona- tremia, sodium Introduction Carbamazepine is an anticonvulsant and psy- chotropic medication commonly used for the treat- ment of a number of clinical conditions in patients both with and without intellectual disability (ID; Sovner & Hurley ). The side-effects of carba- mazepine include hyponatremia (BMA & RPSGB ). The putative mechanisms for this have been well explored by Kastner et al. () and Spigset & Hedenmalm (). The reported incidence of hyponatremia during carbamazepine therapy in patients with ID varies from % (Kastner et al. ) to .% (Lahr ). It has been suggested that the incidence of carbamazepine-induced hyponatremia increases with age and with an increasing serum carbamazepine level (Lahr ). Carbamazepine is widely used in the treatment of patients with ID. It is used for an increasing number of conditions, including epilepsy, bipolar Journal of Intellectual Disability Research     pp –   ©  Blackwell Science Ltd Hyponatremia during carbamazepine therapy in patients with intellectual disability B. D. Kelly & J. Hillery Stewart’s Hospital Services Limited, Dublin, Ireland Correspondence: J. Hillery, Consultant Psychiatrist, Stewart’s Hos- pital Services Limited, Palmerstown, Dublin , Ireland.