Objective: To evaluate atypical electroencephalographic features in subacute sclerosing panencephalitis (SSPE)and to detect its relation to clinical features. Methods: Twenty-two patients aged 2-17 years (mean 9.4 years) with definite diagnosis of SSPE were studied. Their clinical data and EEG records were reviewed retrospectively. All EEG records were analysed for features of periodic complexes (PCs) in relation to age, age at onset, clinical stage and the rate of progression as well as duration of the disease. Results: Classical periodic complexes of SSPE were found in EEGs of 13 patients (group I). Atypical patterns were observed in EEGs of nine patients (group II). Two new atypical findings were identified: prolonged discharges which include sharp waves and slow waves for 4-7 s followed by suppression for 1-4 s; and periodic complexes which consist of four or five sharp waves in every 2 s. We observed atypical EEG patterns were more frequently in Stage III, acute form, and the disease duration was longer than in the typical group. Conclusions: Atypical EEG patterns in SSPE might be related to the progression of the disease, but this theory needs further longitudinal studies. Significance: We suggest atypical EEG patterns might be observed more frequently in patients with severe neurologic disability, more rapidly progressive disease and longer duration of disease. Objective: To evaluate atypical electroencephalographic features in subacute sclerosing panencephalitis (SSPE) and to detect its relation to clinical features. Methods: Twenty-two patients aged 2-17 years (mean 9.4 years) with definite diagnosis of SSPE were studied. and EEG records were reviewed retrospectively. All EEG records were analyzed for features of periodic complexes (PCs) in relation to age, age at onset, clinical stage and the rate of progression as well as duration of the disease. Results: Classical periodic complexes of SSPE were found in EEGs of 13 patients (group I). Atypical patterns were observed in EEGs of nine patients (group II). Two new atypical findings were identified: prolonged discharges which include sharp waves and slow waves for 4-7 s followed by suppression for 1-4 s; and periodic complexes which consist of four or five sharp waves in every 2 s. We observed atypical EEG patterns were more frequently in Stage III, acute form, and the dise ase duration was longer than in the typical group. Conclusions: Atypical EEG patterns in SSPE might be related to the progression of the disease, but this theory needs further longitudinal studies. severe neurologic disability, more rapidly progressive disease and longer duration of disease.