While preeclampsia is common in pregnancy, associated hyponatraemia is rare with very few cases reported in the literature. We report the case of a previously healthy nulliparous woman who presented at 34 weeks' gestation with hypertension and proteinuria. On admission her serum sodium was 122mmol/L and by day 6, in the absence of fluid restriction, it had fallen to 116mmol/L. Urine and plasma osmolalities suggested a syndrome of inappropriate antidiuretic hormone secretion. She was delivered on the sixth day by caesarean section because of fetal distress and worsening preeclampsia. Postoperatively fluid intake was restricted and her sodium normalised within 48h. Preeclampsia results in a low effective circulating volume which can cause a non-osmotic release of antidiuretic hormone and a resultant increase in the urine/plasma osmolality ratio to greater than 1. In patients with preeclampsia, hyponatraemia may further increase the risk of seizures and should therefore be closely monitored and treated without delay.