Abstract Aggravation of seizures due to hyponatremia was investigated in five patients with epilepsy and polydipsia–hyponatremia. They experienced marked increases in the frequency of their complex partial seizures with a decrease in the serum sodium level to 118–127 mEq/L. In all cases, the serum sodium level returned to normal through restriction of fluids, and the clinical seizures improved. All patients had shown intellectual impairment and/or psychotic episodes, and all had been given antipsychotics. Hyponatremia caused by polydipsia appears to be a risk factor for aggravation of habitual seizures in patients with epilepsy.