In order to determine whether or not there is any correlation between the various aspects of hypsarrhythmia and the etiology, we studied one of the major components of this pattern, delta activity, in patients with infantile spasms. In 3 different etiologic groups of patients (prenatal, perinatal and cryptogenic) we divided this activity into 2 types: diffuse (DSA) and focal slow activity (FSA). These two activities were quantified in 3 consecutive minute periods of wakefulness, drowsiness and slow sleep. The mean values of DSA and FSA remained unchanged during the 3 consecutive minute periods and in the different stages of vigilance. DSA and FSA differed significantly according to the etiology, with a predominance of DSA in the cryptogenic group and of FSA in the prenatal group. DSA was not correlated with either sex or age. The focal component seems to be related to brain lesions, whereas the diffuse component appears to be as a stereotyped neurophysiological phenomenon independent of brain lesions, sex or age.