The video encephalograms (EEGs) of 77 consecutive infantile spasms patients were evaluated for the presence of focal or asymmetric hypsarrhythmia and infantile spasms, to determine whether these findings were useful in predicting the presence of focal structural brain disease and were of any additional diagnostic or prognostic significance. Of the 77 patients with infantile spasms, 38% had focal or lateralized features present on video-EEG studies. Unilateral hypsarrhythmia and asymmetric ictal EEG changes during infantile spasms often occurred together: each always indicated the side of a focal or asymmetric structural cerebral lesion that was visible on computed tomographic or magnetic resonance imaging brain scan and was usually large. Clinically asymmetric infantile spasms were less common, always occurred in the presence of asymmetric ictal EEG changes, and did not appear to have additional localizing value. Lateralized hypsarrhythmia, with or without asymmetric infantile spasms, occurred in the presence of bilateral structural lesions that were more abnormal in the area of the greater EEG abnormality. Partial seizures also indicated symptomatic etiologies but were less localizing to visible focal lesions. Patients with symmetric hypsarrhythmia and infantile spasms rarely had focal/lateralized lesions visible on imaging studies. Although the majority of the symmetric group had structural brain disease, these brain lesions were diffuse, not lateralized. This group also included all patients who had cryptogenic etiology and normal development.