PURPOSE: Fever plays an important role in causing disturbances in the fluid and electrolyte balance, especially in an immature brain. Recently, it was reported that hyponatremia enhanced the susceptibility of febrile convulsions in children and increased the risk of repeat convulsions during the same febrile illness. We studied the relationship between hyponatremia and febrile convulsions. METHODS: Blood samples for electrolyte measurements were taken from 52 children who visited Chosun University Hospital between June 1997 and July 1998(patient group I: simple febrile convulsion, patient group II: repeated febrile convulsions). We checked serum electrolytes in groups of age-matched controls(control group I: no fever, no convulsion, control group II: fever only, control group III: non-febrile convulsion). The results were analyzed by Student's t-test and ANOVA test. RESULTS: The mean serum sodium level of both patient groups(136.8+/-2.3mmol/L) was significantly lower when compared to all control groups(control groupI: 140.7+/-2.5mmol/L, control group II: 139.7+/-3.1mmol/L, control group III: 139.7+/-4.6mmol/L)(P<0.05). The mean serum sodium levels were not different between each of the control groups(P>0.05). The mean serum sodium level in the group with repeat convulsions(136.8+/-2.4mmol/L) was not significantly lower than the mean in the group with simple febrile convulsions(136.7+/-2.2mmol/L). There was no statistical relationship between the level of serum sodium and the probability of repeat convulsions(r=0.19, P>0.05). CONCLUSION: Our results show that low serum sodium concentration may increase the risk of febrile convulsions. However, there is no statistical relationship between the level of serum sodium and the probability of repeat convulsions. These findings warrant further studies on the relationship between the control of electrolyte levels and seizures.