Autism may develop in children with West syndrome. This study was conducted to determine if EEG abnormalities in patients with West syndrome predict the later onset of autism. Two groups of patients with West syndrome, older than 6 years of age, were studied. One group consisted of those with a past history of West syndrome plus autism (N = 14); the control group consisted of those with a past history of West syndrome but without autism (N = 14). Patients were followed at regular intervals and video-EEG recordings were done. A total of 108 (autistic group) and 123 (non-autistic group) video-EEGs were examined. The two groups were compared with respect to age, presence or absence of hypsarrhythmia, and characteristics and localization of the epileptogenic foci. chi(2) and Fisher's exact tests were used. The number of patients with at least one hypsarrhythmic EEG at the age of one year or later was significantly higher in autistic subjects (86%) than in non-autistic controls (29%). The incidence of EEGs with hypsarrhythmia was also higher in the autistic group, especially in older children (autistic, 49% versus non-autistic, 18% at age 3 years and later). Frontal predominance of the primary foci on EE-Gs with or without hypsarrhythmia was seen in 95.3% of the autistic group and 28.8% of the non-autistic group (p = 0.001). Frontal abnormalities on the EEGs, which were mainly bilateral, and the persistence of hypsarrhythmia were significantly related to the emergence of autistic behavior in patients with West syndrome. These findings suggest that paroxysmal discharges in the cortical areas undergoing rapid maturation may be involved in the development of autistic features. (C) 2006 Elsevier B.V. All rights reserved.