In September and October, 2004, an outbreak of encephalopathy of unknown etiology occurred in certain areas of Japan including Yamagata, Akita, and Niigata prefectures. These patients had a history of chronic renal failure, most of them had undergone hemodialysis, and also had a history of eating Sugihiratake (Pleurocybella porrigens), an autumn mushroom without known toxicity. Since clinical details of this type of encephalopathy remain unknown, we analyzed the clinical, radiological and electroencephalographic (EEG) features of ten cases of this encephalopathy in Yamagata prefecture. The summary of the present study is as follows: 1. Ten patients had chronic renal failure, and seven underwent hemodialysis. 2. Each patient had a history of eating Sugihiratake within 2-3 weeks of the onset of neurological symptoms. 3. The onset was subacute; the initial symptoms were tremor, dysarthria, and/or weakness of the extremities, which lasted an average of 4.5 days (ranging from 2 to 11 days), followed by severe consciousness disturbance and intractable seizures, resulting in status epilepticus in 5 patients. Myoclonus was also seen in 4 patients and Babinski reflex in 3. 4. Brain CT and MRI examinations were unremarkable in the early stages of the disease. Three to eight days after onset, however, conspicuous lesions appeared in the areas of the insula and basal ganglia in 6 patients. On MRI, these brain lesions were hyperintense on T2-weighted and FLAIR images, and hypointense on T1-weighted images. 5. EEG examination was performed in 6 patients, all of whom showed abnormal EEG findings. Periodic synchronous discharge (PSD) was seen in 2 patients, spike and wave complex in one patient, and non-specific slow waves in 3. 6. Prognosis was different from case to case. Three patients died at 13, 14, and 29 days after onset. Two patients still showed persistent disturbance of consciousness one month after onset. One patient showed parkinsonism after recovering from consciousness disturbance. Four patients recovered nearly completely around one month after onset In 3 of the 4 recovered patients, renal failure was not severe and they did not need to undergo hemodialysis. This suggests that the degree of renal failure is a key for the prognosis of this type of encephalopathy. The present study suggests that this endemic disease is a newly recognized clinical entity of encephalopathy.