An outbreak of acute encephalopathy among 6 patients with renal dysfunction after eating "Sugihiratake" mushroom in the northern area of Niigata Prefecture was reported. All of the patients had varying degrees of renal dysfunction, and 3 of them were on dialysis treatment. Patients initially presented with shaking limbs and difficulty in walking. Several days later, disturbances in consciousness developed, which were followed by status epileptics. Four patients were put on mechanical ventilation for seizure control. Two of the 6 cases died. CSF examination showed elevated protein without pleocytosis; bacterial, rickettsial and viral screenings was negative. Brain CT and MRI studies showed edema in the claustrum and external capsule, and in the white matter of the base of the frontal, parietal, and temporal lobes. No common drug administration was recognized. All of the patients had a history to have ingested "Sugihitarake" mushroom (angels' wing mushroom, pleurocybella porrigens) in varying quantities and frequencies prior to the onset of the illness. This mushroom may have induced toxic encephalopathy although no similar case have been reported in the past.