We report a 75-year-old man with a ten-year history of parkinsonism, who developed acute delirium. He was well until 65 years of age, when there was an onset of tremor in his left hand. He had been treated with levodopa and trihexyphenidyl with marked improvement. He was doing well until July of 1991 at his age of 75-year-old, when he noted backache; he was found to have a compression fracture of the third lumbar spine. He had to use a wheel-chair most of the time since then. In December 1991, he started to have visual hallucinations and difficulty in swallowing. On December 25 of that year, he developed fever and delirium, and was admitted to our hospital on December 30. On admission, general physical examination was unremarkable except for low grade fever (37.3 degrees C). Neurologic examination revealed an alert but chronically ill patient. Apparently he had visual hallucinations. He was disoriented to all spheres; he could respond only to simple questions. Apparently he was demented. Cranial nerves were intact except for mumbling speech disturbance and masked face. He was unable to stand or walk. He had flexion contracture in his both knee joints. He had paratonic rigidity in all his limbs and marked bradykinesia. Once in a while, myoclonic jerks were seen in both upper and lower extremities. Deep reflexes were diminished bilaterally, and the plantar reflex was not elicited on either side. On laboratory examination, BUN was increased to 72 mg/dl, creatinine to 3.0 mg/dl, and WBC 16,000/microliter. Cranial CT scans were unremarkable.(ABSTRACT TRUNCATED AT 250 WORDS)