This report concerns 2 autopsy cases of Creutzfeldt-Jakob disease (CJD) in which the relationship between changes in serial electroencephalograms and the course of the disease is analysed. The importance of timing when periodic synchronous discharge (PSD) occurs was indicated. i.e., the appearance of PSD paralleled clinical manifestations of the apallic syndrome and was most distinct when the disease was at its worst. We reviewed the EEG findings in autopsy cases of CJD reported in Japan. Fifteen cases of typical PSD were found and these were examined clinico-pathologically. The findings were: 1. PSD was recorded within three months of the onset of the disease in two thirds of the 15 cases. 2. In 13 cases the clinical manifestations when PSD occurred were those of apallic syndrome or of similar conditions. 3. Following characteristics were found in the distribution of pathological changes in the brain; (a) The presence of extensive changes in the cerebral cortex was the only finding common to all cases. (b) The cerebral changes mainly occurred in the corpus striatum of the basal ganglia and in the medical nuclei of the thalamus. (c) Disturbances were least in the brain-stem below the level of the midbrain. (d) No definite tendency was apparent in the changes of the cerebral white matter and cerebellum. In conclusion, the mechanism of PSD may be accounted for by disturbances of the cortico-thalamic circuit. This circuit is said to be regulated by diffuse thalamic projection system. It is presumed that the extensive pathology in the cortex and medical nuclei of the thalamus plays an important role.