BACKGROUND: Delayed postanoxic encephalopathy (DPE) is a demyelinating disorder in which cognitive, behavioral and movement deteriorations relapse in a few weeks after a complete recovery from initial hypoxic injury. To our knowledge, there have been no reports describing the temporal change of diffusion-weighted imaging (DWI) findings in DPE. METHODS: In a patient with DPE, serial brain MRI including DWI was performed at admission, 1, 3, 5 and 11 months after the onset of DPE. In order to obtain apparent diffusion coefficient (ADC) value of the lesion, we selected the same axial slices of the serial DWI and placed eight regions of interest (ROIs) in the periventricular white matter. Mean ADC values of ROIs and ADC map were serially compared. RESULTS: Compared to normal value, the ADC values of ROIs remained low at admission and one-month follow-up (0.68+/-0.08 and 0.67 +/- 0.08 X 10(-3)mm2/s) and then continued to increase on the 3, 5 and 11-month follow-up (0.78+/-0.05, 0.80+/-0.05 and 0.87 +/- 0.11 X 10(-3)mm2/s). Abnormal low signal density in both periventricular white matters on initial ADC map also disappeared slowly. Serial ADC maps revealed continuing diffuse cytotoxic edema over 1 month after the onset of DPE. CONCLUSIONS: Delayed cytotoxic edema associated with delayed clinical symptom of DPE might implicate that possible pathogenesis of DPE is apoptosis. DWI including ADC map might be useful to detect apoptotic brain lesion.