We reported a patient with subacute sclerosing panencephalitis (SSPE) in whom EEG had been serially performed before the onset. She was referred to our hospital due to epileptic seizures at 2 years of age. Focal spikes were seen on EEG at the time of her first seizure (2 years 4 months). At the time of the second unprovoked seizure (9 years 8 months), EEG showed poorly organized background activity associated with focal spikes and a few diffuse spike-and-waves. Then, diffuse paroxysms became predominant, followed by periodic synchronous discharges. In our case, EEG abnormalities were recognized before mental deterioration. Unexpected EEG changes in a patient with epilepsy could be a clue as to the diagnosis of SSPE.