Purpose: To investigate the nature of edematous lesions seen on MR images during acute episodes of hypertensive encephalopathy(HTE) with particular attention to the findings of diffusion-weighted imaging (DWI) . Materials and Methods: A total of 17 MR examinations in fourteen patients with hypertensive encephalopathy were performed, The diagnoses were idiopathic HTE in eight cases, eclampsia in three, and cyclosporin-inducted HTE in three. The apparent diffusion coefficients(ADCs) of edematous lesions and normal white matter revealed by DWI were assessed and compared, and the changes observed at follow-up MR imaging were analysed. Results: DWI obtained within one week of the appearance of acute neurological symptoms revealed the ede-ma as iso-intense in all patients with eclampsia and cyclosporin-induced HTE, and in five of eight patients with idiopathic HTE. In the other three patients with idiopathic HTE, DWI demonstrated slightly hyperintense erie-ma. The ADCs of edematous lesion in patients with idiopathic HTE, eclampsia and cyclosporin-induced HTE were 1.21 $pm$0.34, 1.08$pm$0.28, and 1.28$pm$0.22 mm$^2$/ms, respectively, while for normal white matter the corre-sponding figures were 0.77$pm$0.25,0.71 $pm$0.22, and 0.68$pm$0.27mm$^2$/ms The differences in ADCs between erie-ma and normal white matter were thus significantly different between the three patient groups (p < 0.05),while the ADCs o( edematous lesions showed no sisgnificant variation between these groups (p< 0.05). follow-up MRI revealed that in three cases, edematous lesions were reversible and there were no residual signal changes. Conclusion: Vasogenic rather than cytotoxic edema is present during the acute stage of HTE.