The classification of the background EEG was performed in 132 fullterm neonates who had suffered perinatal hypoxia by visual inspection of whole polygraphic records of 2--3 h duration, i.e., maximally depressed, markedly depressed, moderately depressed, mildly depressed, minimally depressed, normal and other, according to the distribution of 7 EEG patterns. The quantitative analysis of the polygraphic record was made in 33 of the above and 10 control infants to investigate the effect of perinatal hypoxia on the background EEG and behavioral state cycle and to confirm the validity of the above classification. The calssification of the background EEG was well correlated with the disturbance of sleep cycle, which was also related to the interval after the hypoxic insult. The relation between sleep states and EEG patterns also had a close correlation with the classification and became progressively disrupted with increasing severity of the background EEG. Each grade of the background EEG abnormality had a different prognostic significance accordsing to the time of the recording. The one in the first week offered the best prognostic value.