Clinical and electroencephalogram (EEG) observations permit a hypothesis for the pathophysiology of infantile spasms. The triggering area(s) for spasms is probably cortical. Diffuse hyperexcitability of the immature cortex and/or diffusion of the epileptic process from a focal abnormality would allow development of hypsarrhythmia. Cortical maturation from posterior to anterior might account for the location of causative focal abnormalities. In cryptogenic West syndrome, the predominant pathophysiologic process may be diffuse cortical hyperexcitability, explaining the severe developmental deterioration at the onset of spasms. In symptomatic West syndrome with focal pathology, the initial deterioration is often less striking, suggesting that the predominant effect is diffusion of the epileptic process. The pattern of eventual cognitive deficits follows a similar pattern.