Between September and November 2004, idiopathic acute encephalopathy occurred in many patients mainly living in Niigata, Yamagata, and Akita area in Japan. Almost all patients had end-stage renal disease (ESRD) and/or had eaten a kind of mushroom (botanical name: Pluerocybella porringens). Twenty-four cases of idiopathic acute encephalopathy were diagnosed in Akita Prefecture. Of these 24 cases, 17 patients were treated with maintenance dialysis and three patients with ESRD were in the pre-dialysis stage. The present study analyzed the clinical backgrounds and dialysis conditions of 17 dialysis patients to identify factors related to the occurrence of acute encephalopathy. The mean patient age was 64.2 years, nine patients were men, the mean dialysis duration was 56.8 months, the main underlying diseases causing ESRD were diabetic nephropathy and chronic glomerulonephritis. Patients who consumed the mushroom and underwent hemodialysis in common. There were no other common factors in the clinical backgrounds of these patients involving drugs or dialysis conditions, such as dialysates, anticoagulants, and dialysis membrane. Furthermore, we investigated the incidence of mushroom consumption and development of acute encephalopathy in 102 dialysis patients at 2 two dialysis centers. Of 102 patients, 57 (55.8%) had eaten the mushroom, 5 of 57 patients (8.7%) developed acute encephalopathy, and 2 of these 5 (40%) died of acute encephalopathy. We failed to find any common factors in the clinical backgrounds or dialysis conditions related to the occurrence of acute encephalopathy other than undergoing hemodialysis and consuming the mushroom. Further investigations are needed to identify the cause of this acute encephalopathy epidemic.