SummaryClinical, polygraphic and pathologic descriptions were made on a 54 years old female case of relapsing form of CO poisoning who survived for 4 months with “apallic syndrome”.Special attention was paid to the sleep cycle pattern of the “apallic” patient and 24 hours polygraphic recordings were made to get the quantitative data on it. Since the low voltage theta EEG pattern did not show significant change in waking, drowsy and REM sleep periods, the sleep‐wakefulness cycle was determined by the level of EMG activity. It was revealed that the diurnal sleep‐wakefulness rhythm was not involved profoundly in the apallic state as far as the REM sleep is concerned, though the rhythm itself became irregular to some extent. The abrupt shift from the state of wakefulness to REM sleep was laso observed. These changes in the sleep mechanism were explained from the widespread interruption of the cerebral cortical function.The pathological changes consisted of diffuse demyelination of mottled appearance in the entire cerebral white matter with small softening in the deeper area of the frontal and occipital lobes. The U‐fibers were spared. “Island of preservation” of myelin as well as the spongy area was noted. Reparative replacement by fibrous gl