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-induced 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 edema 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 edema. The ADCs of edematous lesion in patients with idiopathic HTE, eclampsia and cyclosporin-induced HTE were 1.21 +/-0.34, 1.08 +/-0.28, and 1.28 +/-0.22 mm 2 /ms, respectively, while for normal white matter the corresponding figures were 0.77 +/-0.25, 0.71 +/-0.22, and 0.68 +/-0.27mm 2 /ms The differences in ADCs between edema and normal white matter were thus significantly different between the three patient groups (p<0.05), while the ADCs of 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.