Fifty cases of idiopathic epilepsy and 25 age and sex matched normal healthy controls were studied. There was a significant fall in serum magnesium levels in idiopathic epilepsy on day one and 4 of the seizures (P < 0.01) as compared to controls, and was not decreased significantly on day 7 of the seizures (P < 0.05). The fall was maximum in status epilepticus and severe epilepsy than in mild and moderate epilepsy. Serum magnesium took less time to come back to normal in mild and moderate epilepsy than in other types. This shows that serum magnesium does have a prognostic significance. Lesser the fall in the magnesium levels, less frequent were the convulsions and vice versa. It is concluded that serum magnesium continues to be a sensitive early diagnostic index of idiopathic epilepsy.