Posterior reversible encephalopathy syndrome (PRES) is a potentially devastating neurologic syndrome, but timely treatment may lead to complete reversal of the disease course. We reviewed 12 cases of PRES and describe the clinical history and imaging findings, including conventional magnetic resonance imaging (MRI), diffusion-weighted imaging (DWI), and calculated apparent diffusion coefficient (ADC) maps, used to establish the diagnosis of PRES. Three male and nine female patients aged between 11 and 70 years (mean, 37 years) with clinical and imaging findings consistent with PRES were enrolled in the study. All patients had undergone conventional MRI and 10 had undergone additional DWI studies. Ten patients had follow-up MRI studies. DWI was performed using a 1.5T system with a single-shot spin-echo echoplanar pulse sequence. Initial and follow-up neuroimaging and clinical history were reviewed. Lesions were almost always present over the posterior circulation, mainly the parieto-occipital region, affecting primarily the white matter. The anterior circulation region, brainstem, cerebellum, deep cerebral white matter, and thalamus were also involved in five cases. Conventional MRI revealed hyperintensity on T2-weighted and fluid-attenuated inversion recovery images. DWI showed isointensity and increased signal intensity on ADC values in all cases, indicating vasogenic edema. Clinical and MRI follow-up showed that the symptoms and radiologic abnormalities could be reversed after appropriate treatment of the causes of PRES in most patients (9 of 10). In one patient, the ADC value was lower on follow-up images, indicating cytotoxic edema with ischemic infarct. DWI was a useful complement to MRI in the diagnosis of PRES.