We report of 51-year-old man with early onset parkinsonism. The patient was well until 38 years of age, when he noted a difficulty in the use of his right leg; this difficulty improved after he received a medicine from his physician. He did not take medicine regularly, and he noted difficulty in standing up from a chair and in rolling over at age 40. Tremor was not a feature, but he noted slowness in his movements at age 42; at age 49, he noted diurnal fluctuation in his symptoms and at times he experienced hallucination. He was admitted to our hospital in September of 1992 for the first time when he was 50-year-old. At that time, neurologic examination revealed an alert and somewhat bradyphrenic man; Hasegawa dementia rating scale was 20/30. Cranial nerves were intact except for masked face and small voice. He showed stooped posture and small step gait cogwheel rigidity was noted in the four limbs more on the left; tremor was absent. Deep reflexes were within normal range and the sensation was intact. As he showed diurnal fluctuation in his symptoms, his medication was switched to levodopa 3,000 mg/day without a peripheral decarboxylase inhibitor. He was discharged for out patient follow up. But he did not take drugs regularly, and his neurologic condition deteriorated; he was admitted to another hospital. Neurologic examination at that time was essentially similar to that of his first admission to our hospital, except that he showed more severe rigidity and akinesia; again tremor was not detected. His cranial CT scan showed a mild ventricular dilatation without cortical or brain stem atrophy. During his hospital stay, he developed episodes of oculogyric crisis during peak dose of levodopa, and orthostatic hypotension. He developed pneumonia and expired on October 28, 1993. He was discussed in a neurological CPC, and the chief discussion arrived at the conclusion that the patient had early onset Parkinson's disease of Lewy body type. As differential diagnoses, early onset parkinsonism without Lewy body, pure form of diffuse Lewy body disease, pallidoluysian atrophy, and other conditions were considered; however, all of those possibilities were excluded. Early onset parkinsonism without Lewy body would have much earlier onset than this patient, and diffuse Lewy body disease would show more profound dementia 13 years after the onset. Pallidoluysian atrophy would be complicated with some dystonic features. Post-mortem examination showed marked discoloration and degeneration of the substantia nigra. The degeneration was most prominent in the ventrolateral tier of the substantia nigra.(ABSTRACT TRUNCATED AT 400 WORDS)