Cerebrospinal fluid (CSF) and serum lactic acid levels were prospectively assayed in 42 children less than 5 years old with febrile convulsions who were divided into two groups for analytical purposes, irrespective of aetiology of pyrexia. One group (24 children) had brief febrile seizures and the remaining 18 children had prolonged febrile seizures. CSF and serum lactic acid values were obtained for brief and prolonged seizures. On admission, the mean CSF lactic acid was significantly higher (p < 0.05) in children with prolonged than with brief seizures, but the mean serum lactic acid was not significantly different between the two groups. Twelve of the 18 children (67%) with prolonged seizures and elevated CSF lactate had seizure recurrence. There were no recurrences of seizures in children with a brief initial seizure and low CSF lactate. CSF lactic acid is elevated in children with prolonged seizures and this may be useful in detecting those with a prolonged seizure where the history of the duration of the seizure is not clear. Prolonged seizure has been associated with the risk of recurrence and therefore such children with elevated CSF lactic acid levels might benefit from long-term anticonvulsant therapy.