We report a case of a reversible posterior leukoencephalopathy syndrome (RPLS). A 57-year-old male had classical polyarteritis nodosa with mononeuritis multiplex, renal insufficiency, and a high titer of p-ANCA (MPO). He was normotensive. He was treated with high dose methylprednisolone and then with oral prednisolone and cyclophosphamide. Despite the treatment, his renal function rapidly deteriorated and hypertension progressed. He had two generalized seizures; at that time his blood pressure was 200/140 mmHg. CT scan revealed bilaterally symmetric hypodensities in the thalamus, the occipital white matter, and the brainstem. T2-weighted MRI showed increased signal intensities in the temporo-occipital white matter, the thalamus, the posterior limbs of the internal capsules, the external capsules, the midbrain, the pons, and the middle cerebellar peduncles. T1-weighted images showed hypointensities in these areas. Treatment with nifedipine improved his blood pressure; 5 days later he was only moderately disoriented. Follow-up CT demonstrated an ill-defined hypodense area only in the left parietal lobe. To our knowledge, there are only two reported cases of RPLS associated with systemic vasculitides. Interestingly, thalamic lesions are outstanding also in these cases. Under these circumstances, treatment of hypertension is of primary importance, and steroid therapy should not be withdrawn or reduced.