PURPOSE: Recently hyponatremia was reported to be common in febrile convulsion and related to the probability of a repeated convulsion. It was documented that hyponatremia lowers the threshold for convulsion in experiment animals. We therefore studied the relationship between hyponatremia and febrile convulsion. METHODS: Serum sodium concentration was measured in 82 children with febrile convulsions and 66 age-matched febrile controls without convulsion from January 1995 to September 1996 at Ewha Womans University Hospital. The relationship between hyponatremia and febrile convulsion, including the repeat or recurrency was examined in 17 children with repeated convulsions during the same febrile period and 36 children with recurrent convulsions during the other febrile illness. RESULTS: Serum sodium concentration in febrile convulsions was 134.9 +/- 2.3mEq/L which was significantly lower than 138.2 +/- 2.4mEq/L in febrile controls (P<0.01) and that in repeated and recurrent febrile convulsions was significantly lower compared to non-repeated and non- recurrent febrile convulsions. The incidence of hyponatremia in febrile convulsions was 36 of 82 children (44%) which was significantly higher than the incidence in febrile controls (3%) (P<0.01) and that in repeated febrile convulsions (59%) and recurrent convulsions (56%) had significantly higher incidence than that in non-repeated (31%) and non-recurrent febrile convulsions (37%) (P<0.05). The increase in probability of a repeated and recurrent convulsion when serum sodium level decreases, was significant (P<0.05). Conclusion : Serum sodium concentration in children with febrile convulsions was significantly lower than febrile children without convulsions and the lower the serum sodium concentration, the higher the probability of a repeated and recurrent febrile convulsion.