We studied the prognostic applicability of electroencephalograms (EEGs) of seventy-nine patients within 24 hours after successful cardiopulmonary resuscitation. The EEGs were classified into five grades according to a modified Hockaday's scale. The EEGs of grades I and II implied full recovery, while those of grade III gave a varied but generally unfavorable prognosis. Patients with grades IV and V EEGs survived in a vegetative condition or died without awakening. Eighteen patients showed EEG with periodic patterns, all of which led to a fatal or vegetative outcome. One case showed EEGs associated with periodic triphasic waves and repetitive sharp transients in the same record. Several cases showed EEGs with different periodic patterns in consecutive records. We conclude that an EEG is a good indicator of patient prognosis after cardiopulmonary resuscitation. However, the clinical significance of morphological differences of various periodic patterns that can occur during an EEG remains to be established.